You have to ask yourself a few questions here in order to make decisions...

How does their friendship make you feel aside from that?

How much does that behavior bother you?

Do you feel they do it malitiously?

Have you mentioned to them that it bothers you?

Do you do things with one or the other that may bother the third party?

How much are you willing to put into this friendship?

Do you think you are overly jealous of their friendship?

Is this your only group of friends?

I don't tend to put alot of effort nito friendships that don't lift me up....

Those are my 2 cents worth.

Yeah, these are the kinds of fights you have when you are five and six.. because then people still ring in your mind as property, and friends "belong" to you.

In reality, people tend to worry more about themselves than stuff like this. I think you should tell them that it's completely boring and juvenile, and then I think you should recommend some Shel Silverstein books to facilitate the next step in their psychosocial development.

I don't think they do it to be malicious, I just think they're rather immature.