I have a pretty good one-hander but have been trying to transition to a two-handed over the past ~6-8 months. I think my two-handed is progressing overall well, but one issue I noticed is that my right shoulder (I am a right) moves up during the swing, while looking at pro footage, their right shoulder seems to stay almost level and well below their left shoulder. Any tips for how to go about solving this? Thanks!
Hard to know exactly what's going on with your two-hander, but looking at that snapshot gives me the impression that you could still be hitting your one-hander with your other hand (left hand) along for the ride. I say this because I think that the right shoulder is more prone to moving up like you're describing when hitting a one-hander compared with a two-hander.
I've always had a much stronger aptitude for hitting a one-hander myself, but I've also worked a good deal on building a useful two-hander for certain occasions. What's been a big help for me and also for many of the young sluggers I've coached is to work on letting the other side - your left side - be more in charge of the two-hander. This can be tougher than it sounds, especially if you instinctively trust your one-hander.
The drill I like to use for working on this is to take a two-handed bh grip, remove the bottom hand (your right hand), leave the other hand in that "choked up" location on the handle, and take several practice swings to find a comfortable tempo. Then hit some left-handed forehands with that same left hand location on your handle. These aren't real shots, so don't get hung up on where they go. Just find a comfortable swing to decent contact that includes pushing off your left foot similar to how you push off your right foot to drive your righty forehand. If you're using that wall in the pictures for drills, you can hand feed yourself with your right hand to get some left-handed reps.
If your left side is more in charge of your two-hander, your left hand will probably want to sort of roll over the top of your right hand as you release the racquet through contact. When that happens, your shoulders will probably want to remain more level or your right might slightly drop as you follow through with your hands and wrists progressing left-over-right. It might also feel like your shoulders want to rock from left to right (tilt left-up-right-down) as you swing.
One more tricky aspect of the two-handed backhand: If we approach it like it's an off-handed forehand, its best contact point will probably be further back than what works best with the one-hander. Try to experiment with different contact points while focusing on that left hand being the primary driver of the racquet for your two-hander. This is something I need to routinely brush up on to keep my two-hander in half-decent working order.
Sorry for the novel, but hopefully there's a nugget or two in here that you can put to work.