They work in CrossfireX, and use the PCIE slot, not an additional hardware bridge.
In general, you would be better served by a single, more powerful card than two relatively weak ones. Yes, I've seen some benchmarks putting CrossfireXed HD7750s at or above a GTX560Ti, but I'd bet that microstutter would be very noticeable. Better to spend the same amount (or a little more) on the GTX560Ti or HD7850. Make sure your PSU is up to the task; although a good 450W PSU would probably be sufficient, I'd recommend 500W for an additional margin of safety and some headroom.
Thank you for the reply and the advice. Why do you suppose Newegg lists every one of their 7750s as 'no crossfire'? The previous model; HD 6750s were listed as crossfire. Do you suppose someone at Newegg looked at the cards and saw no bridge header, and assumed no crossfire?
Thanks. As to your last question, it is hard to say what someone at Newegg thinks. There's Paul, often featured on their videos, who seems to know what he's doing, but then you can be sure there are a lot of marketing droids there (as at any company), who are more likely to be the ones responsible for writing copy for product descriptions.