What you describe is bi-amping, not bi-wiring. Bi-wiring is when you run two sets of wires from the speaker to a single amp/receiver. With bi-amping you have one amp/receiver driving one set of drivers and a second amp/receiver driving the other set of drivers.
It sounds like you have both the Fisher and the Marantz running at the same time using the passive crossover in the speakers. I assume you have the bass driver (terms 1 & 2) connected to one receiver with a pair of wires. Then you have a third wire connecting the tweeter (term T) to the plus speaker output of your other receiver. Because your receivers have a common ground and you are connecting them together by sharing the same input signal, your speakers work. But the negative speaker output from the receiver connected to the tweeter is flowing through something that does not normally see speaker power.
The idea behind bi-amping (and bi-wiring) is to have the speaker current carried by independent wires between the amps/receivers and the speaker drivers. In your case, the tweeter receiver has the current for the tweeter common flowing through a non-traditional path. That non-traditional path may be the chassis ground connected by your power cords or it may be the shields of your interconnect that ties the two receivers together. Don't do that.
Your tweeter amp/receiver needs to have two wires for each channel connected to the tweeter circuit. You should connect a pair of wires for each channel to the tweeter (terms 1 and T).
PS: Wiring two amps/receivers to the AR this way will fry an amp/receiver that does not have a common ground. Fortunately for you, it worked out.