It is equal parts risky and ambitious to release a double album – especially in today’s music-buying society.

Spreading out songs over two records, and not condensing down to only the essential tracks, causes holes in an act’s song-writing to become more noticeable. Listeners are forced to switch back and forth between releases, instead of having all the material in one place.

In this post-Napster, singles-oriented environment, it takes a band either brazenly confident or foolishly arrogant to package enough music to fill two discs.

Enter System of a Down.

By splitting its double-album into two individual releases six months apart, the oddball metal quartet succeeded where other recent acts with that approach failed. At no point does “Hypnotize” play like leftover tracks not good enough to make the first release “Mezmerize.” (See Radiohead’s “Amnesiac” and “Kid A”).

It plays like a cohesive record with two complementary parts.

For each catchy, radio-ready tune on “Mezmerize,” chalked full of stop-and-start hard rock (“B.Y.O.B.,” “Cigaro”), there’s a theatrical and epic tune on “Hypnotize” (“Tentative,” “Lonely Day”).

The two balance each other out. The series never sounds too similar or seems like the same record. But the two albums aren’t distinctive enough to be worlds apart.

What SOAD has done is figure out how to get a fickle record-reluctant public to accept two albums worth of material – they just need to be Hypnotized and Mezmerized.