This is a strange question to ask, as it's not conventional logic in how the cards work.
Basically they both will work at the same speeds they do now... HOWEVER. The faster card will finish it's rendering quicker than the slower card, which forces the faster card to 'wait' to display what it has rendered. Following me here?
The slower card is the benchmark for the faster card, so there is no real reason to overclock if you can't get both of the cards at the same speed, at least, in a Crossfire solution.
Also, you will get a total of 512mb addressable space for Crossfire. Think of the two cards as one, instead of on a PCB they are together via a soft 'bridge'.