AGoAC rulebook, page 48.
Basically, Player 1 picks a pilot card for one of his mechs. Player 2 upgrades one of his mechs to equal pilot/gunnery skills(not SPAs), then chooses a card for one of his other mechs, whose SPA cost is equal to(or less if they want) the cost on Player 1's card. Player 1 now upgrades one of his non-card mechs to match the skills of Player 2's card pilot. In terms of pilots, the match is now considered roughly balanced.