Hello, I am sure there is a better way but you COULD try using a splitter on each of the headset output plugs. You would probably need a splitter that replicates 1/4" jack sockets. You could then use a 1/4"->3.5mm adaptor. Then take the two 3.5mm leads and use a coupler lead to combine the two mono channels into one stereo 3.5mm lead. Then you can plug this into your MP3 player and use the recording facility. The bonus of using this method is that (assuming your MP3 can record stereo) is that each channel will be recorded individually, therefore you can split the recording into two sides easily afterwards.
The specifics of this method might be different (i dont know enough about std aviation comms) but it should work nicely. As I said, there maybe better ways, but this method should work and provide a higher quality recording than using other microphones (as these will probably not be as good quality as that on your aviation headsets). Hope this makes some sense. All this should be available on ebay for low $$$