I suspect it's marking territory too. I notice this with our humpy bunny, he'll always go to the litter tray right after Poppy to sniff at her pee and chin her poos/the edge of the tray.
I wonder if it might be worth removing their shared tray (either giving them separate ones you swap out when you open the house up or removing entirely - they'll go back to their own room if they need to go).
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