Congratz! Then, my advises:
1. Keep and feed them, they're good. Combine for skills+slots aren't increase her power too much this time, u can think about it next 1 year.
2. If u still want to do it, u need to wait for buy a guarante bloom (100% skill success chance).

Keep both of them, but don't spend any resources except a handful of lvl 5 manyu's on one of the two, upgrade the other as usual. As soon as you have 20 girls stronger than the "weaker" Baccara, she fulfilled her purpose and gets to merge with her big sister. Should be relatively cheap, but efficient.

thanks for your answers I think I'll keep her for now

I'd probably keep them both since the odds of getting a better slice girl any time soon are slim to none.

Personally I'd keep them and have them in different teams, sure one may never get equip ups or skill ups, but she'd stiull be kickass