http://treasureadventureworld.com/pre-order/ Starbound's preorder page has a (slightly) discounted 4-pack option. I assume this exists because it incentivizes people to go out and convince their friends or others online to bundle up for the game, increasing sales and awareness across the internet. Shouldn't there be one for TAW too? Like, I don't know: Treasure Adventure World 4-Pack - $36 (10% off!): 4 preorders of the game for $9 each.
That's a good idea. I'll have to find out how that went for Chucklefish. It may also be that a multiplayer game like Starbound is more likely to get people who buy it for their friends. I'm going to look into it, though
As RobitStudios said above, SB is multiplayer. The multi-packs are not something any single-player game ever really does.