A power diode in each leg of the Y would isolate the two devices from each other. Direction of diode depending on polarity of voltage being keyed.
I have been pretty lucky using a "Y" connector to share PTT around the shack. I do remember a similar situation with my station and I think I got out easy by reversing the key-up wires of the device that did not like the "Y".
And the diode approach might be a better engineering approach