That's a pretty solid way to do it, for sure. Splitting the action economy allows them to do some unique positional stuff but generally keeps them limited.
The shared HP Pool is interesting, too... though I think I'd probably split it between the two rather than giving them a shared value with the caveat that they can share hit points through physical contact. Maybe make it so that during a short or long rest they can decide who has how many hit points. That way you could have a "Tanky" one and a "Glass Cannon" for different playstyle options of the same heritage.
That would help offset the "One of them falls into a pit and the other one dies for no apparent reason" and give them the ability to maintain activity when one of the twins is at 0hp, but also makes them individually fairly frail, to encourage them to avoid staying in melee combat and to hide or take cover whenever possible.