Well now I feel kind of dumb. I've never looked at the info part of the game page. I think at some point I saw that combining the sets allowed for a six player game and did the math that six players minus four players = two player expansion. I don't think I would ever actually want to play this way, but the follow up question here is, why can't you combine the sets for an eight player game?
Desinger tested wrote and proposed rules for combining the two games with 5 and 6 players. It is possible to play with 7 and 8 palyers with 2 sets , but probably it is better to play 2 concurrent 2-4 player - cards not chosen in one game are palyed in the second game.