I haye a well established female cat from a rescue home who is extremely nervous. I have been asked to home another and wondered if they would share a common toilet tray. I have a large garden which my cat spends nearly all day in.
From my limited knowledge I think it is always best to have more than one tray if you have more than one cat as they are unlikely to want to use the same one. A rule I read somewhere was that you should actually have one more tray than the number of cats you have as some don't even like to do both in the same place so 1 cat = 2 trays, 2 cats = 3 trays etc. My one cat has 2 trays but he only ever uses one of them so not sure how accurate that is - probably depends on the cat but I would certainly recommend having at least 2 if you have 2 cats.
i do some voluntry work in a rescue cattery and ther are several pairs of cats there who share single litter trays, without any fuss. if your cats are going to spend most of the day in the garden they may not want to use a litter tray much, so one would probably do them both.