Hello. Unfortunately you can't do that. You can only sli/crossfire the same cards. It doesn't matter if the cards are different brands (ex. Asus & MSI) you can sli/crossfire them, but they have to be the same card.
EXAMPLE
Asus GTX 770 & MSI GTX 770 = OK
Gainward GF 9600 GT & Asus GTX 550 TI = Can't