You wouldn’t split into two transactions if you want to avoid both Alice and Bob being able to spend xG+4H. Two separate transactions is a straw-man with drawbacks as you are pointing out.
The two transaction variant is doable today as long as Alice and Bob are willing to live with the drawbacks. It is not what I am discussing above.
The single extended transaction I am discussing, { }, cannot be implemented with the protocol today. The protocol today has no provision to re-blind.
But you realize this cannot be split into separate transactions because either-can-spend outputs are problematic and an invitation for cheating.
Is there a non-obvious reason your extremely anti-this?
I don’t see a need for a transaction that needs all 3: trustless, spotty connection, and instant. You can design for any pick 2 and there would be reasons to do so; you could just make a side protocol for “friends and family” transactions and design merchant focused wallets to just never ever except it