Is it possible to run one of the newer huge 11-42 cassettes on a 10 speed double chain ring setup?

Yes, but only if your rear derailleur can handle the capacity. Your big chainring + big cog gear combo requires more chain length than the small chainring + small cog combo. Your chain should be long enough to work in the big+big combo, which will leave some amount of slack when you're running in other gears. In a conventional rear derailleur design, the cage of the derailleur swings back to take up that slack. The longer the cage, the further back it can swing, and the more slack it can take up.

Rear derailleurs have a "chain wrap" capacity that tells you how much of a gear range they can handle. Figure out the difference in the number of teeth on your biggest and smallest cogs, and the same for your chainrings, and add them together. That's the wrap capacity your derailleur will need to handle. If you're running an 11-42 cassette, that's a 31-tooth difference. If you're running a 36-46 cyclocross crankset up front, that's a difference of 10. Add those together and you'll need a derailleur with a wrap capacity of at least 41. There are some mountain bike derailleurs that can handle that capacity, but you'd be exceeding the capacity of most road derailleurs -- even long cage models designed for triple cranksets.

You may end up exceeding the take-up capacity of the rear derailleur. It's still workable in that case. The chain should be sized to work in the large/large combination so you don't cause damage if accidentally shifting into it. The chain may end up being slack when in the small ring and the smaller cogs but that won't cause damage and those combinations can be avoided.

Thanks for info guys. I think if I ever do this I would have to diligently remember that big-big is off limits, even accidentally. I figured to get the chain sized comfortably on what would be a 48-36 front/rear combination (big-second big) and just make sure I was in the smaller ring when I went to the 42. Current setup on the bike is 48/34 up front with a 11x34 cassette, 10 speed. Rear dérailleur is a shimano R350, which I believe is med cage.

No, you have the wrong idea about this. You HAVE to make the big-big combination workable. It doesn't matter how much you try to avoid the combination, you will eventually shift into it. I had my bike setup exactly as you describe before, it was very, very annoying. Having to constantly worry about accidently overshifting and tearing off your derailleur. If you're going to do something like this, you'll need a long cage derailleur. That's what they're made for. (To take up lots of chain.) It doesn't make sense to set it up so that you can't shift into big big when the other way (can't shift into small-small) won't cause any damage whatsoever. (It'll just rub the chain and make lots of noise.)

I'm running an internal gear crankset .. chainring still one but it turns at 2 different rates relative to the crankarms.

so in this theoretical situation the single ring can act like 2..

Patterson Bike - Rocket Fast Shifting | Cannot Throw a Chain is one ...

That looks super expensive.

Point taken. I'm just trying to keep cost's to a minimum but do understand what you are saying.

Rebuying your drivetrain to use those 10-42 cassettes aint cheap either.. stick to what you have already.

“The best laid schemes o' mice an' men / Gang aft a-gley.”- Robert Burns
You would need a new derailleur to be able to handle that 42 tooth cog anyway, and such a derailleur would have enough take up capacity to handle the extra chain length, so there would be little point in sizing the chain that way. But no matter what you do there will be that time when you will forget that you are already on the 48-34 combination and go for the easier gear. The consequences could be unpleasant

I thought 10 speed and 11 speed cassettes used different size chain links.

I am sure there must be derailleurs out there which can handle the 42 cog---1x11 bikes must use Something. And I would bet, in time, someone will come out with an Xtra-Long Cage rear derailleur; supply comes from demand, and really all it would be would be to add a fraction of an inch to two derailleur cage halves.

Cost, on the other hand ....

Seems to me it would be a lot cheaper and simpler to go to a triple chainring.

There are two issues.

Take Up Capacity. Basically how long of an arm your derailleur has.
Sprocket Clearance. Whether the derailleur smashes into the large sprockets when shifting.

You can avoid cross chaining all you want, but your derailleur still won't work if it is not designed to physically reach those larger sprockets (and not have the jockey wheels rolling along the top fo the rear sprockets).

I see a lot of notes about changing pull ratios, but I am having troubles finding the exact specs for the new 11s road and MTB derailleurs.

And a hell of a lot nicer to ride. The jumps between gears on a 11-42 must be hideous.

Then you got to change cranks, shifter, front mech. Too much money.

Sunrace make a 11-42 10 speed cassette. I think the jumps are acceptable by most accounts. Its a big last step up to the 42 but by that time who cares.

What hills are you climbing that you need a 11-42 in the back?

It might be easier to swap the front chainrings for a subcompact rather than changing out your drivetrain to accommodate the giant cassette in the back.

Actually it's the steps at the bottom end that would concern me. 11-13 is close to a 20% jump. 15-18 is a full 20%. Nasty, in my opinion. Much prefer a triple and nice tight ratios at the back.

The idea of the 11-42 rear cassette is that you only need one front ring. If you have a 34 T rear just go to a 36 T cassette. Then you have the 2 chainrings up front to get the gearing you need. What current chainrings are you using? Lowest GI trying to achieve?

What about the longer hangers that allow the derailleur to clear the larger gears? I don't know anything about the OP's derailleur but I picked up a hanger extension that is suppose to allow my old Shimano derailluer to take a 40t and I think they say the MTB ones can handle 42t (Wolftooth Roadlink or Goatlink). I plan on experimenting to see if I can run a 11-40 with a DA7800 RD. I hate using the front gears, they make the ratios overlap and also more fiddly to shift, I'd rather have all my gears in the back and perfectly sequential (less to setup and tune too). It is an experiment though, we'll see how it goes I may end up not liking it!

The further you get the upper jockey wheel from the cassette sprockets, the more chain flex you will get, and less precise shifting.

Maybe some math.

Circumference = πD = 2πR

Add 10 teeth (5") to the circumference, and you add about 1.6" to the sprocket diameter, or about 0.8" to the radius.

So, that means you would need to drop the derailleur down by a little over 3/4" for every 10 "extra" teeth beyond the normal derailleur range. I suppose not too bad.

In regards to hills I would say none but one. I race in the Paris to Ancaster 70km gravel grinder (kinda) every year. The course is okay but the last hill is a absolute bugger. It's in the last half kilometer and it breaks my heart every year. I have a personal vendetta against that bastard. I dont want to use the age card but I am getting up there, so I need all the mechanical help I can get.

Like it wasn't complicated enough I got another question on this subject. Last year I put a new SLX long cage MTB rear mech on my winter beater. It has that goat link thingy build right into it. The winter beater is a 9 speed but would that mech work on the 10 speed cassette?

It should. No telling if it will.

On second thought, it should Not.

Cassettes respond only the the amount the cable pulls them. Given design parameters (width the thing can actually swing) any derailleur should work with any cassette. The deciding factor is shifters.

If you have 9-speed Shimano road shifters, they likely will not work with a ten-speed MTB stuff. Different length of cable gets pulled. (You can possibly buy a ShiftMate to even things up, however.) If you have 10-speed MTB shifters, you should be golden.

If the SLX rear derailleur is 9-speed (SIS), then it will index a 10-speed cassette perfectly when shifted with a 10-speed Road (SIS) shifter. Nashbar has the 10-speed bar-ends for $50, btw.

If the SLX rear derailleur is 10-speed (Dyna-Sys), then it will index a 10-speed cassette perfectly when shifted with a 10-speed MTB (Dyna-Sys) shifter.

The extra "goatlink" hanger may make the indexing a little bit off on one end or the other of the cassette, according to some reviews. I haven't tried one myself.