Re
"This is making 2 completely unrelated projects dependent - which is not ideal."
My understanding is that when A resources enters time the act through date is set for all the project where the resource is assigned (not allocated) might recall the discussion where that was posted.
Dave_3.0 might recall the discussion where that was posted.
Secondly the usual way of extending tasks where you have a problem like that is to remove the assignment ETC, set the new end date and re-apply the ETC.