99.9% of all radios and scanners use the same wiring schime. center is + out side is ground.
If you make your own adaptor, then you can also add a smal blocking cap in line with both sides to give you piece of mind.
I have several radios and scanners that I quite offen 'patch' together thru one headset and have never had to put in cap's or had any problems with hearing what was going on on either device.
If you make your own, then do it this way: stero plug on one end for the headset, center of one mono cable to tip of plug, center of another mono cable to the ring side of plug, combine the two grounds and connect to the ground side of the headset plug.
Now you have a 'split' cable that feeds single audio from radio 1 to one side and radio 2 to the other.