I really dont know Bryony, I have a cat thats never pooped in the tray and he's nearly 6
Even when there was just him and his sister, he would wee in the tray and then go through to the kitchen and poo on the floor, he still does that to this day
He has stoped pooing in the bath tho, which is a good thing I suppose
All I could suggest is, maybe put the other tray beside the one thats already there and buy a plastic mat to go underneath them to save the carpet.
There has even been a time when Ted (the floor pooper) has just walked into the living room and just pooped on the floor right infront of me, I was like what the... I couldn't believe my eyes