The Tur quotes at length the opinion of the Rambam that you can't make an eiruv chatzerot on YT which is erev shabbat [you obviously don't need an eiruv chatzerot for YT and it's in hilchot YT because of making it for shabbat on YT], but then says the Rosh said it's ok and the SA paskens like the Rosh and allows it if there's a tenai.

OK, that's fine.

But to make an eiruv chatzerot on YT, even with a tenai, do you need have made an eiruv tavshilin first on erev YT since you're basically preparing from YT to shabbat?

According to the ones that you can do so without a tanai, you can do so only if you made an eruv Tavshilin, but I think we base the heter on the tanai, that it has nothing to do with an eruv tavshilin, this is the impression I got from only slightly glancing at the various opinions, but afterward looking at the Aruch Hashulchan 529:2 I get the impression that that is what he is saying.