Tapping the See Results button displays the actual list of songs: The two-paned window shows tracks unique to the first library on the left, with tracks unique to the second library on the right. As you scroll through the lists, Mashduo lets you jump directly to a track’s entry on the iTunes Store to purchase it—for example, if you really were comparing your library to a friend’s, and you found something in your friend’s library you wanted to buy. (Mashduo doesn't let you preview tracks within the utility, or go to the actual track in your iTunes library; I'm guessing this is because you can perform your comparison on any computer—it doesn't have to be either of the computers hosting the two libraries—so the tracks may not exist on that computer.) You can also export either set of unique songs to a text file, or email the results.

Armed with the information I needed, I went back to iTunes on my wife’s old PC, selected each of the songs that were unique to that library but missing from her Mac. I dragged all of them from iTunes to a folder on the PC, and then transferred them to my wife’s Mac. One feature I’d like to see here is for Mashduo to let you create a standard playlist file for, say, tracks unique to Computer 1. You could then import that playlist into iTunes on Computer 1, making it much easier to grab those unique tracks.

Finally, back in the warm, cozy confines of OS X, I dragged the missing music into my wife’s library, sang “Happy Birthday,” and rejoiced in her happiness at my recovery of long-missing tunes. (I also got her flowers and other presents. And a nice dinner. But I digress.)

Though I didn't need them, Mashduo packs in a few advanced options, too. For example, you can limit its comparison to songs matching (or matching) a specific genre, limit it to comparing an individual artist, and tweak how it performs song comparisons.

Mashduo is one of those niche apps that serves one specific purpose, but does it very well. I’d gladly have paid for the app, since it made such quick work of what could have been a tedious task. At the low, low price of free, it’s even better: a simple, effective tool for comparing iTunes libraries in an instant.