Can a player be on more than one team (multi-team players)?

Yes, if the organizer allows it, a player can belong to more than one team within the same league. This is controlled by an organizer setting for the league, so by default a player usually sticks to a single team, but the organizer can switch on multi-team players when it suits the format. This is useful for casual or short-format leagues where the same people fill out several squads. Outside of a league that has this enabled, a player normally represents one team per competition. Check with your league organizer if you are unsure whether it is turned on.