If someone gets his name second on a paper despite the two contributing the same amount of work and this fact is indicated so, 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.
The two authors contributed equally to this work
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)