2 Radios plugged into 1 Speaker?

67Dart273 said
In my opinion you are on very dangerous ground here. Depending on "what radios" he has, the audio output may be "single ended" or required to be above ground "balanced." Additionally, any failure in either output stage could take out components in the other radio

I see where you're going here and you're right but if he went with a double pull double throw he could switch both positive and negative between each radio. The only other danger he could run into is if the dash speaker had a permanently grounded negative lead.