Well just tell her that you think she is attractive and would love to take her out to dinner sometime. Ps. try to just be yourself, and your going to be nervous so just get it over with.
Not at the same time no.
I've heard that if you're 'seeing' someone you can 'see' someone else though.
If you're wanting to do this, you should talk to the other two people involved.
If she's not into a relationship with another girl, then it just won't happen. Some people are gay/bi, which obviously helps, and some people are just open-minded. But if they don't like the idea,...