While games of Warhammer typically take place between two people, there is nothing stopping you from playing games with allies. To forge your alliances, split the players into two teams. The teams do not need to have the same number of players, but if you are playing a game that uses points values it is best to give a little thought to the relative sizes of the players' collections when assigning teams, so that you can be happy that the balance of the game is as you wish it to be. The players can now pick their armies and begin to play.
Alternatively, if both players agree, you could choose to pick your force from two different armies. Ideally this should be done by both sides so that each player is given an equal chance to build an allied force.
Regardless of the number of players, each army must have their own General and Battle Standard Bearer.