Originally Posted by cxwrench

I'm sure people have 'made it work' but it's definitely not correct. The 812 will work w/ the 42t cassette, but it's for a 1x crank...it doesn't wrap enough chain for a double. The 810 derailleur does wrap more chain, it's capacity is 41t, but it will only work w/ a 34t cassette. Same w/ the RX derailleur. You might be able to get a 36 to work but a 42 is wayyyy past the spec of both those derailleurs. Adjusting the b-tension screw so the derailleur will shift to an overly large cog will mess up the shifting on the small cog side of the cassette as the upper pulley is too far away to shift accurately.
It would be great if people understood derailleur specs in general and capacity in particular before posting advice like the above. This thread should also be posted in 'bicycle mechanics' for obvious reasons and not GD.