Ok so I wanted to record some things on garageband and adobe soundbooth but the Macbook only has one Line-in and one Line-Out. I thought about it for a bit and decided what I would do. I ended up cutting some wires and sticking some together to create a circuit that allowed this to happen. It's really simple, basically get your 2 input instruments (mono devices such as guitar or microphone) and join the 2 mono signals into one stereo signal. This plugs into the standard line-in and preserves the 2 signals. Go into garageband or soundbooth (or whatever you use) and select 2 tracks, One recording from mono line 1 and the other recording from mono line 2. (from the stereo line in). There you have it, yes you are relying on the onboard pre-amps and stuff but for most people it just works like that, simple and good enough for quite a few people including me. I was thinking about making a little very basic product out of the idea. pretty much just an adapter. 2 mono inputs 1/4" jacks and one stereo output 1/8" jack. Put it in a neat little box and there you have it.