My guess would be that if they're using Bidan stand-ins for events, too many people would know when those stand-ins check into the WH.
If there's a set across the way, and tunnels with multiple entries, stand-ins could be brought in undetected more easily..
So, say you're staff or a guard at the WH and Bidan is reported as being in the WH. Why would "Bidan" show up at the gate or enter the building? If you have the other "Bidan" enter the set from a tunnel, which has more than just the WH access door and the double used that , it wouldn't be suspicious