If it is stated that two authors have contributed the same on a research paper, are both authors co-first authors?
There is only one first author. But the 'prestige' associated with the first author, in certain fields, could be extended to a second author. 

An example:


  The two authors contributed equally to this work


(source: https://arxiv.org/pdf/1404.7748.pdf)

When cited, this work would be "Najman and Cousty" ("Najman"/"Najman et al" in some contexts, if more than 2 authors), but it is clearly stated that both authors contributed equally, so they both share the 'main author' credit.

(this is the only example I know of this approach, they were my phd advisors - yes, both of them)