Warhammer is normally a clash between two mighty armies, striving for dominance upon their chosen battlefield, but this doesn't mean that it is a game solely for two players. By combining several players' armies into alliances, it's possible for any number of players to join the battle. Games involving allies will often trigger all sort of bizarre alliances, depending on the armies available to the players. You may wish to invent a narrative to explain the reasons behind the alliance, or simply decide not to worry about it and get stuck in the game. It's entirely up to you.
To forge your alliances, split the players into two teams. The teams don't need to have the same number of players, but if you're playing a game that uses points values it's 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.