I dont think excluding from articulation would help here.
The franka arm is a fixed based articulation, its root fixed joint has to be disabled (not excluded). Then you should select the franka root body and the moving robot body and connect it with a new fixed joint.
Then the whole connected hierarchy would be solved as one articulation. If you require to have still two articulations created in PhysX, then this new fixed joint between the articulations have to be excluded from the articulation. Hope that helps.
Regards,
Ales