I do interlinked control systems, multi-circuit SWA cables etc and is all perfectly acceptable as long as certain criteria is met and in OP case it fine as others have mentioned that the joint has appropriate labling warning of multiple circuits, anyone gaining access will need to follow correct isolation and testing measures before altering or working on the joint, failure to do so cant be blamed on the fact it has an uncommon situation for a domestic install, whether previous bodgit or sparky has labled it or not if isolation and test procedures are followed to work on the joint then it will not be a safety issue. Those who get complacent or assume will learn a big lesson - if not on the job in hand then a future job.
When ever i alter, extend or rip out others work i always assume they were incompetent and always tread carefully; id most likely be dead now if i didn't judging by my finds over the years.