All you need to do is install both cards into your motherboards PCI-E lanes, Connect the crossfire bridges that come with the cards (You can use 1 or 2 bridges, it doesn't make any difference). Boot into windows, install the latest drivers from ATi's website, make sure you enable Crossfire in CCC.
Thats about it really.
(Obviously you need a crossfire compatible motherboard and a quality 600W or greater power supply.)
Why not check out some HD5770 Crossfire reviews and see how well they scale together.
EDIT: Did some googling for you: http://www.guru3d.com/news/radeon-hd-5770-review-crossfire/