I know of no way to do what you are asking for on the PBX alone. If you had CTI and a lot of programming you might be able to do what you are asking, but you would end up chewing up 2 trunks for ever call that was not handled but the local PBX’s call group. (if you transfer a call of site at the PBX level the PBX will act as the bridge.) If you use the Carrier, they will charge you something per call, then the call is sent back to the carrier, and frees up your trunks.
Hope you find a solution.