You know what would be almost functionally equivalent. Release bitcoin2 as soon as it becomes a problem. it could be literally exactly the same as bitcoin1 in every way except 2.
Not entirely, the main/root chain effectively secures all the value for all the chains in the hierarchy.
You could also have software that can handle the promote/demote system efficiently.
now at this point you have something fairly similar to what i originally outlined, only instead of actully moving btc from one chain to the other, you could just use seamless electronic exchanges.
You might even be able to merge the signatures in some way. The transaction would have to be valid in both chains.