Note that scores and opinions vary about which of Mercedes and Frasquita should sing which line. I have followed the commonest understanding, which is that were one line sits predominantly lower than the other, that will be Mercedes, despite what the score says. Basically, as soon as you are cast in this role, get in contact with the singer taking the part of Frasquita and sort it out between you!
Note (January 2019): I have now included a separate file (at no extra charge) with an alternative part swap in the card trio. On reflection I think this makes more sense, with Frasquita being the flirty one, generally sitting higher in practical terms.