A: its only rude if you leave one person out of the conversation completely you have to acknowledge them both and keep them both in the conversation. Don't irritate the girls you have to have a purpose to your conversation, what is it you want from them? Knowing your purpose and stating it to yourself will not only make the conversation easier but the approach will be more natural and you will feel more confident.
B: There is a lot of confusion about what a neg is and I think you have a different idea as well. Look at it as a way of NEGating the idea you are attracted to girl2. If you treat her like an after thought (not ignore her) during the conversation she will be more relaxed as you clearly aren't hitting on her. the fact you couldn't even see her helps this even more so, it was a boon in your favor.
If you have seen the movie crazy stupid love, just remember the scene where he tells a girl she is boring after everything she says, thats using a neg incorrectly

and its just rude at that point and it becomes an insult.