May I inquire as to your gender? The reason I ask is because if you are male, it could be that your gf's don't quite understand your insecurities as they probably don't see how that either one of them can replace what you bring to the relationship. This might lead them to not completely understand how to honor your concerns.
A lot of times stating specific needs (behaviors) as opposed to non-specific needs is helpful. They probably have very little idea how to help you work through your concerns, even if they are totally in favor of doing so. They are unable to read your mind. Your gf's may be frustrated because they don't really know what you want, feel helpless to solve the problem, and also feel deprived of each other due to work schedules, etc.
So what specific things could your gf's do to help you?
And while it know it is tough, they will probably be a lot more able to be compassionate if they don't feel like they have to fight for some time alone. Maybe if you could find a way to give them your blessing (even if it means setting a schedule), they won't feel so frustrated.