side team

Being the fan of two different sports teams in the same sport often not in the same division as to not be on the losing side.

An indecisive sports fan.

Who will you root for if the Knicks play the Mavericks? I know one of them is your side team.

He is cheating on his main team with his side team this tonight.

Having a side team means that no matter who wins the Super Bowl, you will always be on top.