Hi
Cats like to have their own litter tray as they do not like to smell another cats urine as it is used a s territory marker. It could be that if you only have one tray they are each marking the area as their own. Then the other cat comes along and urinates in the same area to try and cover up the other cats smell.
I would recommend two separate litter trays in different parts of the house. This way they have their own separate territories and should not feel the need to urinate around the area. :)
