As far as i know, two identical cards means same model with same graphics memory, same bandwidth, same speed and all...
in your case both 5670 or both 5770, not 5670 and 5770.
Once Cilus told that a HD5770 & a HD5850 can be Xfired. also the HD5850 won't be clocked down to HD5770 speed. something of that kind. so i suspect, its possible to Xfire cards of same gen i.e. 5-series with 5 series or 6-series with 6-series. i'll do a bit of goggling & report back here if i find anything.
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