In such a case, the specific identity of the sire is socially unimportant- you are a part of the pack that sired you, and that's enough information. The result are always Caitiff (known as Pander). Notably, the sabbat are known to create new childer using the mixed blood of a group of vampires.
![embrace of the vampires embrace of the vampires](https://i.ytimg.com/vi/fFw5fZGrl5Q/mqdefault.jpg)
There's wiggle room if you need it- the Curse is not very thoroughly understood, and its most widely-known, observed properties are based on relatively thin generations in fairly narrow points out in comments that something very near to this situation is documented: I'm thinking specifically of Augustus Giovanni, who sort-of-but-not-quite-totally diablerized an Antedeluvian, and as a result became one himself mostly-but-maybe-not-quite-all-the-way. There are a few cases of vitae working differently in specific cases than is typical, so there's some precedence for special rules if you like. To be first, even if I flip a coin to figure out which one that is.
![embrace of the vampires embrace of the vampires](https://cache.moviestillsdb.com/i/500x/wwjnrqi1/embrace-of-the-vampire-lg.jpg)
If one vampire Embraces a mortal, that vampire's vitae causes the transformation and that vampire is the sire. Mechanically there isn't much ambiguity here, as there is (generally) no combination siring. Kindred society under the Camarilla holds a sire responsible for their childer, clanless or not, so the "official" sire could be whichever vampire volunteers for the role, both together, or whomever the prince chooses. Getting permission to sire is a big deal, and two individuals getting together to use their individual permissions to produce a single teamwork-childe would be an unusual way to expend it. If you're talking about socially, that's an odd situation that I doubt the Camarilla has standing rules for. If it's exactly simultaneous, or the vitae is mixed, you're likely to get a clanless result. The one whose vitae causes the transformation.