If you put two of the same gpu cards in (crossfire) does this double the Hash rate?
Yes it does.
I am using 2 5750s, which combine to a Hashrate of 300 MH/s.
Also if I am not mistaken: In order to use them for mining you do not even need the crossfire bridges. Of course in that case (without the crossfire bridges) you will not be able to benefit from them when gaming (should you plan on doing that).