Thank you for all your reassuring replies and advice!
Chewy and Oreo are both so much happier now! I did a lot of research but somehow missed bonding a pair. I will only spot clean a bit at a time from now on. I'm attempting the litter training but so far one ignores it and the other kicks it everywhere. Now they are living apart should not share toys when out to play because of scents?
XRubyRosex - That's shocking to tell you to just switch them! I was surprised by some of the advice they gave us!