How to deal with being friends with adult twins and their jealousy towards one another?

I started being friends with one of the twins about a year before the other, and during the time that I was friends with the first twin, there were hardly any issues. After becoming friends with both of them, the first twin that I became friends with became very jealous, and accuses his twin of taking over all of his friends. I will invite both of them and only talk to one twin about it, who will not inform the other twin, so then it comes back on me. When each of them are with me individually they talk badly about each other, and I just feel caught in the middle, and do not say anything, because I cherish both of their friendships. If I invite only one twin to do something, the other one gets mad, and if I invite both of them to do something, one of them gets mad because they don't want their twin going. Now, we are all in our 30's, so you'd think they would be more mature when it comes to jealousy and friendships. I am just wondering if anybody has any input on how to deal with this relationship? Or if it would be best to just get invited to go out and do things by them, and not invite either of them to do things, that way they can't get mad at one another, or feel left out?