A party of two players experience odd behavior in post game. If the party leader backs out before the other player, they are taken to the party view screen (the one that lists the players in the party but this player is now by themselves), as if they weren’t the leader. There is also a scenario where the player who isn’t the party leader can get stuck in the post carnage screen and is never allowed to back out. These issues don’t seem to pop up when the party size is 1 or greater than 2.
Matches can still begin with uneven teams in matchmaking.
Halo: CE games do not end when one team quits out.