System of a Down - Mezmerize/Hypnotize(Do double albums count?)
Not really a double album though as they were released individually, but you're right a double album is what's required so I change my answer to Melancholy and the Infinite Sadness by smashing pumpkins. Plenty of variety and length so you won't get bored on a long trip.
Abbie Road is also a good choice, that meddley on the second side never gets old.