From Populous Wiki
Doubling is the term which players use when two players, usually allied, attack an enemy player at one time. Doubling can be used to a doubled player's team's advantage since one the ally of the doubled player can attack the attacking players' bases while they are both preoccupied. Professional players will make a good defense in their base before they decide to double their opponents. This strategy is used many levels like Pressure Point and Face Off to either finish a player off at the end of the game or to damage their opponent at the start of the game to give them the upper edge.
Doubling is a strategy and should be taken as so, It is not against any rules unless all players decided to not allow it before or during the game.