fatcat, the misbehaving one is female.
cat welfare ppl said try to change type of litter used. when i adopted them, they were using recycled paper pallets, so i simply continued to use the same. i read that some cats will refuse to use anythg other than what they are used to.
but i will try to get somethg else today - maybe the clay type.
i will also give them one tray each, though they are used to sharing.
the thg is she is getting worse these days, now peeing on scratching post and even near their food bowls - YUCKS!!
anyway, thanks, sousa and fatcat for your suggestions.