I've got 2 of the QSPot 150's and I only feel safe putting 1 on each of my speaker stacks in a "normal" have configuration. If I rotate the horn assemblies and go "line array", I would feel no problem going 2 per side, but I only have the 2, so that has a built in limitation. It has to do with size of the speaker.
Make sure you tied down as much as possible. If you are using stands/sticks for mounting your speakers, DO NOT under any circumstances pop lights up on top of your speakers, PERIOD. It's flat out NOT safe.