My daughter (high school freshman, Level 9) trains two-a-day. I have very mixed feelings about it. She started two-a-days after 8th grade--at her insistance. She really wants to be involved in college gymnastics; and she felt that as an "older" gymnast, she really needed the edge that two-a-days could provide.
Obviously, there are the financial aspects to consider--higher gym tuition and private or home school costs. There is also time--do you have time to get her back and forth? Who is going to teach her?
My daughter goes to a private school that caters to children who have outside interests--actors, athletes, musicians, etc. Classes are very, very small (which is good and bad). Between gym and school, her social circle is limited. We have tried to maintain relationships with prior school friends, but it takes alot of effort to keep it up.
On the up side, our daughter now eats dinner with us every night! On the down side, with just two and a half hours of school, there is tons of homework!
Our gym has both one-a-day and two-a-day programs. There is no question that the two-a-days are more thoroughly conditioned, and that they get the gym when it is least busy. However, we have many one-a-day level 10s who score well and go to traditional school. The hard facts are that there is little time for anything other than school and gym whether the girls go once or twice a day--particularly at the upper levels.
I do wonder, sometimes, if my daughter wouldn't know how to quit if she wanted to since school and gym are so intertwined. I wonder if she'll regret later not having a traditional high school experience--no football games, no homecoming, etc. I constantly try to evaluate if she is happy and well-adjusted (no easy task with any teenager!).
It's really a complicated decision--only you will know what's right for your daughter and your family. Good luck!