I usually put them on opposite sides to creat flow through the whole tank.
I've seen fluid dynamic models that show them being in the same corner produces the best circulation. I'll find it later. I run 2 canisters on my 4+ foot tanks with each Filters intake and spraybar in their respective corners. The only area where there isn't good flow is in the very center middle of the tank.
Would love to see it if you do find it!
It's written in German, but google translated it.