I'm pretty fed up with our 3x10 Ultegra drivetrain on our Cannondale Road Tandem 2 (2012).

At the very least, I'd like to go 2x11, and possibly 1x11, in Di2. Some of the roads in our area are 15%+ for a few hundred meters and we bottom out our gearing.

Can I mix an Ultegra Di2 shift lever with an XT Di2 rear derailleur and run an 11-42 cassette? I've read that it's possible with DuraAce Di2 + XTR Di2, but obviously would like to save a few bills.

Yes, the shift lever model does not matter at all, as long as it is of the E-tube variety. With any of these shifters you can drive 1x, 2x or 3x derailleur setups for either road or mtb.

First of all, I'm pretty sure that you'll love the improved shifting from 2x11 Di2.

Before you mix road shifters with XT Di2 rear and front derailleurs, check that Synchro Shift will still work. If my memory is correct, the E-Tube software doesn't allow a mix of road and mtn Di2 components to setup Synchro Shift. Once you select "Road" setup, it will only offer a list of compatible road Di2 components.

You should also consider the new Ultegra RD-R8050-GS rear derailleur. It can now handle a 11-34T rear cassette. If this is still not low enough, you can always get a Wolf Roadlink for a 42T casssette.

You can absolutely mix road and mountain bike components together now. Our adventure Calfee tandem has Ultegra Di2 road shifters and XTR front and rear derailleurs and works flawlessly. I use a XT display on that bike for wireless updates etc. Our road tandem has full Ultegra Di2 with a XTR display and again no issues and they will both do synchro shifting. I have not tried with a different junction box however just the XT or XTR display ones. We have been using the standard ultegra rear derailleur with a Road link or K-edge cage for 11-40 for a couple of years without issues. Also be aware that using a XT front derailleur you need to use a xt rear as at this point you can not mix derailleurs. Also the mtn front have chainring limitations. Both mtn and road shifters are merely switches and are not smart.

I would suggest looking at Sram eTap. Rick at Houseoftandems.com has put together an upgrade kit with a wide range 2X11 with the eTap. I saw a few recently upgraded tandems this past weekend at STR and the owners were very happy with it.

I second eTap with WiFli. Works great and no wires, the best retrofit IMO..super clean.
We have over 2600mi on ours and not one issue with NO adjustments!

https://photos.app.goo.gl/nBtLS7uid6Uhdit03

https://photos.app.goo.gl/V3VyjnrMWVxphyzA3

Beautiful bike!!!!

I notice that your Landshark is setup similar to our Calfee: The FSA triple crank with the big ring (52t in our case) fixed to the middle position and the small ring (34t in our case) attached to the small position. I've been trying to figure out how to go with a 3x system with our Ultegra Di2 but have been warned against. If we switch to an XT or XTR front derailleur, we are pretty severely limited in the size of the big ring (50t?).

I'd like to preserve our range but go to smaller steps between the front rings. Rob at Calfee has cautioned me against this.

Nice. Are those cranks available for a BB30 interface?

Anyone know if I'd be able to mix a regular BB30 righthand compact double crank arm with the existing tandem left rear arm?

There was this report of using a Wifli Etap with 50/34 and 11-36 cassette:
Technical FAQ: Frame sizing, wide gearing for road bikes | VeloNews.com Scroll down...

No evidence of Wolf type extension needed. With etap brifters and RD/FD with batteries and charger now
available for ~$1200, adding a cassette for $60-100 more and a R side compact (I suspect 52-36 would
work as well) would mean a perhaps $1500 conversion.

Thanks, and yes they are available in a BB30

Thanks, its been a phenomenal bike. I will be posting a full write up with photos soon.

So a couple DI2 possibilities that may not be available for Etap. Hydraulic brakes and even greater gear range with some simple mods.