The one whose vitae causes the transformation. If it's exactly simultaneous, or the vitae is mixed, you're likely to get a clanless result.
If you're talking about socially, that's an odd situation that I doubt the Camarilla has standing rules for. 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. 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.
Mechanically there isn't much ambiguity here, as there is (generally) no combination siring. If one vampire Embraces a mortal, that vampire's vitae causes the transformation and that vampire is the sire.
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 you are imagining a scenario in which two vampires are attempting to Embrace the same mortal at the same time and the vitae is somehow exactly synchronized such that neither can be "first", I don't think that there is any clearly-stated, freestanding information published on what happens. Not from 20th Anniversary edition or earlier, at least.
That's Storyteller territory, and I personally would rule something like one of the following:
- Failure. The subject simply dies and does not rise.
- Caitiff. Clanless childer, though not necessarily with the weak blood
that normally implies.
- Perfect synchronization is impossible, and so someone's vitae has
to be first, even if I flip a coin to figure out which one that is.
- Some sort of magical trickery. But when that's involved, rules tend
to disintegrate anyways.
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. 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'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 circumstances.
@BenBarden points out in comments that something very near to this situation is documented:
Notably, the sabbat are known to create new childer using the mixed blood of a group of vampires. The result are always Caitiff (known as Pander).
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.
Mixed blood is about as simultaneous as you're going to be able to get, an so while you can make up some edge cases that might produce a different result (like, a mixture of 99% from vampire A and 0.01% from one hundred others) this is probably the most official answer.