Multiple motion sensor lights on one switch
Unfortunately many or even most of the motion sensor lights on the market are junk.  At least that's been my experience.  I have installed a bunch of these purchased at big box stores that just didn't work, replaced them with better quality brands and had no trouble.  

Before I went to too much trouble with these, I'd remove them and bench test them - wire them up (carefully of course) on a test bench and see if they work as they are supposed to one at a time.  If they don't, you can return them and get something better without wasting a lot of time.  

If they work individually, I'd further bench test that they DON'T work with both on the same circuit.  I am skeptical that that's really an issue.  If it is, it's probably more trouble to rearrange your wiring on separate circuits than it is to just replace them with better lights that don't have that restriction.  

In fact it probably boils down to replacing them either way so maybe it doesn't even make sense to test, just find a high quality brand that works when installed on the same circuit and replace them.