At what point (ovr) do cards start to get double team chems?
i don't think it's an exact OVR threshold, per se -- it's specific to certain / unique card types. If you had some Rookie Premiers, you could run double team chems on 84 OVR, to give an example. This Ji'Ayir Brown has x2 49ers team chems for me right now.
It's somewhat random, but starts to show up later in the year, i'm sorry i'm not better able to address your question.
A lot of the Harvest Promo (usually Champions, 92 OVR) were getting x2 Harvest promo chems, but those also vary a bit. Hopefully others can provide more/better examples of the different ways this works, usually it's done via an additional ability bucket, similar to how secondary positions work on many cards.
Unless I'm mistaken this usually isn't something that's tied to ovr. Usually, if a card gets 2x team chems it's because that's how EA decided the promo works. Team affinity programs like division dynasty are good bets to have double chems.
Other programs like free agency and Ultimate legends have offered players with double chems in the past but those aren't until after the super bowl. But there's no guarantee that's how they will work in this cycle of mut.