UJM
The gemara discusses that we consider if both can be true. Fro example if first pair said they say murder in sura, the 2nd pair says Imanu hayisem in Chicago (Gemara says Nahrdeah but minhag is to use Chicago. Beis din evlauates if can travel between the two in one day. The Gemara even entertaisn the possibility that we consider they used a “flying camel” We don’t. Clearly there is soem form of Derisha Vechakira on the 2nd set. Furthermore the secodn set has to be Yacha lLehazimam as well, which means we need get at least soem basic information from them .
The chiddush, as I understand it is once beis din determines that both groups are equally trustworthy the chidush is we beelive the second pair (and punish the first kasher zamam)
Just my 2 cents, I’m open to hearing an opposing raaya
“The words of the Gemoroh are ‘Kasher zomom velo kasher osso’”
Interestingly there is no such phrase in the Gemara. Rashi and many Rishonim quote it, and the idea is certainly in the Gemara but not that phrase
( I relaize this is an old thread, just find that observation interesting)