Already have a classic album under your belt
This one may seem like a no-brainer, with experience and a dedicated fanbase being two of the definite keys to double album success. Tupac dropped the monumental Me Against The World before All Eyez On Me, Wu-Tang of course had 36 Chambers, Biggie's Life After Death basically hinged on the success of Ready To Die, OutKast had... their first four albums (all classics)-- same for UGK, whose self-titled album is one of the most underrated double discs in history. Nowadays, very few sane major labels would let you waltz into the office and demand a double LP if you don't already have well-respected projects in your discography, but back in the double album boom times, that wasn't necessary.
In 1998, A&M Records let Kurupt make his debut album, Kuruption!, over two hours in length; Houston flash-in-the-pan Lil Flip was allowed to follow up Undaground Legend with the absurdly overstuffed U Gotta Feel Me in 2004; despite the success of his singles, Nelly's Sweat/Suit ended up heralding his downfall from pop stardom.
There are exceptions though, with many of rap's greats failing to bring the same fervor they brought on their classics to double LPs. Jay Z's Blueprint 2 was so superfluous that he released a one-disc version just five months later, and Nas' Street's Disciple is among the worst-received albums of his career. That being said, there have been very few great double albums that have come from relatively untested talents, with Dipset's career-making Diplomatic Immunity being the only one that currently springs to mind.
The Game did follow The Documentary up with a handful of solid projects, and he's obviously been saving the sequel for something very special, but in attempting to match that classic's status while doubling the length, he's still facing an uphill battle.