

If two vampires were for some reason trying to Embrace the same mortal at the same time there isn't any reason to think that the process would work differently- one vampire's vitae will "reach" the mortal and transform them, with the remaining vitae simply providing blood pool. 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.
