Either of Glenn's suggestions should work OK. Personally I like the switch idea since it eliminates the possibility of accidently keying rig A when rig B is the one you want and is hooked into the proper antenna. If you choose the diode option, there is a slight possibility that the voltage drop of a silicon diode of perhaps 0.7 volts is too much and the rig will not key reliably. A Schottky diode, or even many germanium diodes will have much lower voltage drops. I have only run into this problem one time, so not likely to happen. Any of the 1N400x series of silicon diodes should work fine, and cost less than ten cents each.
Rick KL7CW