Punch in/out and crossfades

Actually, it's very easy. Draw a downward "ramp" in one track and an upward ramp in the other one. No art involved.

The other question is if it sounds good. That depends on your material. If you have a melody of single, pretta much separated notes, I would prefer a hard cut between the tracks. I would use crossfade only on long notes that are the same in both tracks, so you don't notice it afterwards.

The other way to make sure that there is a seamless transition when editing together different takes is to zoom in to the max in the editing window and do your cut/paste at a zero-crossing point. That is, in other words, where the line drawing the sound wave crosses the mid-point in the waveform. Paste in the good part and then drag the end of the track to the zero crossing point. Here's a picture: http://img.photobucket.com/albums/v339/jamesbouchard/pastewclick.jpg
http://img.photobucket.com/albums/v339/jamesbouchard/zerocrossingpoint.jpg

There is no real cross-fade and punch-in or out in Garageband, but you can comp together good performances this way without having to use a a lot of fading-in and out on multiple tracks.