Normally, if you take one from DL, then it will be gone from Aeroplan as well. They all come from the same source, unless you're redeeming SQ miles in which case once in a blue moon there are 2 F seats at the same time.

So, even if you try to use 1x DL and 1x AC miles, it's actually just the one single seat that is available, and it's not 1 "per airline".