I think an AR may be more about the consistency boundary than the transaction boundary.
The fact that a transaction happens to fit nicely around an AR boundary is just coincidence.
Since transactions are more of an application layer concern, if you end up with more than one AR in a transaction then it does not necessarily indicate a design issue.
In fact, I would go so far as to say that in some 100% consistency requirement scenarios you may not even have a choice but to include all changes in one transaction.