If the incoming trunk calls came into PBX A, and were normally being routed to extensions on PBX B over a bridge, then perhaps. if trunk calls go directly into their corresponding PBXs, then no. The originator (provider) of the calls would have to instigate the forwarding.
Some VoIP providers will do this automatically. If registration (from the PBX) fails to happen when it should, all calls forward to a new number, that of the other PBX.