They are both 5770 1GB cards so they will work perfectly together. Crossfire also works with dissimilar model cards from the same series. For example a 5850 and a 5770. Although, sometimes mixing models doesn't always give the best yield in performance. You can Crossfire 5xxx cards with other 5xxx card, and 4xxx cards with 4xxx cards, but not mix them.
(2) 5770's perform as good and most times better than a 5850.