Annoyingly the Windows version of iTunes doesn't handle .band projects properly. You can use iTunes to transfer a project from one device into the file sharing area for the app and save it to your computer, but you can't drag a .band folder into that area to upload to another device. In contrast a round trip is possible for iMovieMobile as those project folders are rolled up into a .zip file. I, and others, have submitted bug reports but as yet there has been no change.
The best workaround I've come across is iFunBox. Share the GarageBand project to iTunes on the device in the normal way, connect it to a computer running iFunBox, click the iFunBox Classic button, expand the tree to show App File Sharing > GarageBand and drag the project folder to your computer. Connect the other device, show the App File Sharing > GarageBand view, drag the project folder in, then in GarageBand on the device use the plus symbol in the My Songs view to copy the project from iTunes.
My only slight reservation is that AVG flagged up one of the components of iFunBox as a security risk, although having let AVG take remedial action the iFunBox program carried on working and there is no sign that AVG actually quarantined any files. I suspect a false positive, the program has been around for some time and has plenty of good recommendations. I'm sure if it were known to be malicious I'd have found the evidence.