Ok, fixed that for 5.053. Will get that out soon, first taking a look at a possibly-easy way to make it clearer in the lobby how to actually add a champion.
Thought:
Allow the champion in the P1 slot, even when no other players, and if selected, it automagically adds a "normal" player in the P2 slot?
(or shunts the champion to slot 2)
It makes it transparent and easy, rather than a "you have to add a second player" note.