I joined Haalu first and now I want to join Telvanni.
Is there a way I can join two great houses?

Not without going outside the game, which could have some strange affects to your game. If you recently joined and have a previous save game from before you joined.

Joining a house is a permanent and important decision that shapes the path your character takes. There's no going back. Those are the rules of the great houses.

Sadly, no, there is nothing you can do to change your house now. There's a very "unstable" glitch that allows you to join both Hlaalu and one of the other 2 houses, but this only works if you join Telvanni or Redoran first. You can read about it http://uesp.net/wiki/Morrowind:Redoran_or_Telvanni_and_Hlaalu.

there is a way, as long as you use a pc and not a console. You need to press the ~ key, which will bring up the console. then you type in player->PCJoinFaction telvanni and press enter