Jean-Luc and friends have found a way to make 2 phased transactions atomic (both executed or both not executed).
In a nutshell, one of the problems is that the last approval transaction which should approve both transfer A and B can be front run by the submitter with another transaction to approve just A, then the original transaction would be removed as duplicate. We had a long discussion about ways this can be mitigated. Currently the direction is to rely on account control to prevent this. This means that this specific scenario won't be supported before 1.6.
In 1.5 you'll be able to approve multiple phased transactions with the same approval transaction but the last approver can front run its own approval transaction with a different approval.
After some more brainstorming, I believe we have a working solution that I will implement for 1.5. It will be possible to use phasing to make two transactions of any type atomic (they are either both executed, or both not executed), as long as at least one of them is phasing safe (asset transfers and AE orders are safe). And the good news is that this will not require any additional transactions, just the two that need to be coupled.