We had a similar situation when introducing our new girl, Janni, to our feline family of three about four months ago. After the initial hostiliy quieted down between our new girl, Janni, and our other girl, Gusty, we noticed that someone was leaving us presents in the bathtub every night. With four cats in a household we had to play Sherlock to catch the culprit. Actually we narrowed it down by process of elimination. Some suspects were out on the porch at the time, etc. Anyway, it turned out to be our older girl - who has always been "Miss Clean" about making the litter box.

What then helped was to start giving each one of the two enemies extra attention(lots of petting, brushing, laps, etc. and cheerful pep talks) but to do it in separate quarters like another room with the door closed. That way each could be queen of the moment without the other watching. We noticed after a while the presents in the bathtub ceased. Of course, it also helped to close the bathroom door when we remembered.