Double ending is the process whereby two people talk to each other (usually over some VoIP product like Skype) and each records themselves. Someone then sends their part to the other person who mixes them together with an audio editor like Audacity to produce the end product. The idea is to give the impression that both parties are in the same studio.
If you're into this idea, you should read Andy's entire post, but here's one of the best tips in there as far as I'm concerned:
Use the 1-2-3 clap trick to synchronise the two tracks. When both of you have fired up Audacity (or whatever you are using to record your voice), the first thing you should do, in absolute synchronization, is shout out 3 times in succession, “one… two… three… “ [then clap]. Why. This will produce three very characteristically shaped spiky waveforms on each recording – ideal for aligning the two tracks when you come to merge them. If you get these three spikes lined up, you know everything else is lined up too. It’s a bit like when film makers use the clapper board at the beginning of each scene to synchronise the sound with the picture.
Brilliant! I never knew what that clap board thing was for...
