I agree about reminding them they are individuals. Could you focus on why each school would suit each girl better? Eg explain reasons to your less-linguistic daughter that you feel she would really enjoy one school and why your other seems to fit the other? Maybe say a few words to remind your other DD why she is so special to you (could she be jealous?) and so why you want a school you feel she'd be most comfortable in (listing its plus points)?
I think that you have to try very hard not to voice comparisons - treat them as individuals and talk about their respective strengths and difficulties openly.
If DD2 compares unfavourably with her gifted sister in every area then this is obviously more difficult, as she will resent any insincerity on your part - just tell her what she is good at, and listen to her concerns and fears.
My 10-year old DD is gifted in languages. She is fluent in English, which she often speaks at home, Korean and Mandarin, which she learnt at school, as well as her native Japanese. She is unbelievably talented yet her twin sister has not be gifted in that area. My other DD who is much more comfortable speaking Japanese is far behind her sister at English. My gifted DD has been often a place at a school for the gifted and talented and she will be starting in September but her sister is very much against her going, shouting about why she should be going as well. I'm worried about how to handle this.