Oui, but in English usage, it's way too generic a word for such a specific design. It needs its own buzzword. Trixte, twixte? Esp. since the ones I've seen are not French bikes.

As for pronunciation, the proper French pronunciation would be (anglo-phonically) "meekst". But there is context / usage to consider. Is a Peugeot a "meekst", and a Schwinn a "mix-tee"? Do we drive a Ford "coop", but a Renault "coo-pay"? Do we worship at "No-tra Domm", but study at "Noter Dame"?

(I did overhear a dude at the bike co-op, at a one-on-one workshop, tell a young lady that the bike she brought in was a mixte -- "or, as the French would say, 'mixTAY'." I opted not to spoil his authoritative moment....)