You would need a mixer circuit. Do not combine signals together, you'll burn your audio amps inside radio and vario. Best bet would be to use mixer designed for aircraft (http://www.aircraftspruce.com/catego...diomixer.html), but I'm afraid most of these are designed to drive headsets and do not have enough power to drive speaker. There might be some radios or varios that accept other audio source and have built-in mixer. I installed two indentical speakers for my glider, very simple solution.