Yep! Easy too!
Assuming each of your PBX’s has an external trunk already configured, all you have to do is go to your outbound route that your PBX would use as to make an external call, ad in “Trunk Sequence for Matched Routes” point it to the IAX trunk to your opposite PBX.
If PBX#1 goes to make an external call, and that trunk is full, its going to send the call to the next trunk in this sequence, which will be “PBX#2”. The call is sent to PBX#2 (and assuming your dial pattern matches) PBX#2 will send the all out its external trunk. And vice versa.