Both cards have to match speeds. So the faster card will downclock to match the lower speed one. You can OC the second card to match the first one.
I however would not recommend it. Xfire does NOT double performance, its closer to 60-80% even in the best conditions. Some games dont support Xfire, so you will lose the performance there. They also dump more heat while drawing more power.
Its typically not recommend to Xfire low-entry level cards. I highly recommend a single stronger card. You will get more performance, that scales better to the price.
What is your budget?