Hey! I think we need some pics of the offending kitties 
- I really don't disinfect very often at all. Cats don't like disinfectants or washing up liquids.
If it starts to smell or has some dried poo on it, I will clean it. (Make sure you have a spare for whilst you're cleaning it.
But I do clean the tray out multiple times every day. My two cats have access to outside, but i'm sure they come in to use to the tray 
- We used to have 2 trays for 2 cats. Although we rescued the cats at the same time, they did not know each other and the older boy really didn't like the kitten at first.
We found that they predominately would use the one tray though for wee and the other for poo.
After 6 months we started letting them out during the day (in at night) and at the same time got rid of one of the trays. Not just because they were going out, we just decided that they'd gotten to know each other and were happy to share.
We've had one tray for a few months now and we seem to be doing ok. If we had any problems we could bring out a second again.