Darkman69 :
You must have a cross fire bridge available on the GPUs you want to cross fire. That said I haven't seen a single HD 7750 that has a crossfire bridge. Also Radeon Pro fixes crossfire issues.
Do your homework, bro. 7750 doesn't require a bridge, as the PCIe provides more than enough bandwidth for the 7750s to communicate.
Here's a review of CFX 7750s, specifically the page mentioning that you don't need a CFX Bridge:
http://www.eteknix.com/amd-radeon-hd-7750-crossfire-review/2/