My girlfriend came out to me as a bi...Should I let her pursue a separate relationship or part ways?

There have been a lot of questions here on this subject, so I've done my homework on it. It is possible to make this type of polygamous relationship work. It sounds like you two have a strong relationship and you're at least somewhat ok with being with a polygamous woman. If the relationship means this much to you, it is possible to make it work. Honesty and working through tension and jealousy are very important. I'd really suggest doing some reading on polygamy so that all three of you know what you're getting into and specifics on how to make it work.