So far every answer is right. Yes you can, No, with switches and relays. The technicial answer would be no, not without knowing a lot of other things about the electronic circuits in both radios. If it just something you have to do then the simplest way is to use a double pole double throw toggle switch or relay. Things to consider are, Neither unit should be powered on without the speaker attached, depending on the device it could damage it, so care should be exercised when powering up either radio to make sure the switch is in the correct position. To those that say just hook them both up to the speaker at the same time, I say don't. It might work and it might not depending on the output power configuration of each radio. Even if it does work it will not work at the optimum performance, there will be a impedance mismatch that could result in reduced audio and or distortion. The best way to hook it up would be through a triple pole double throw relay where the two speaker leads go through two of the poles and power to the two radios is routed through the third pole. This would insure that only the radio with the speaker hooked up to it could be turned on. Just my 2 cents.
