I have twin sisters, now all grown up with kids of their own! They were inseparable as kids, and almost always in the same class. This worked for them. Things tend to ebb and flow with twins, they will work most of this out on their own- they will try separate friendships (often because a friend may like one twin but not the other) and either be ok with it or find it just doesn't feel right. Most often, they had the same group of friends. They usually had girls in the group that one liked better than the other and girls in the group usually preferred one twin over the other. Totally normal.
If they have expressed a desire to be together, I would let them. They will feel safer to explore other friendships when they are most comfortable- which is when they are together. Think of kiddos when they are toddlers. They start to want a little independence to explore, so they start to wander away at the playground, but look back often to make sure you are there. Same thing with your girls. This is what is was like for my sisters. They were happiest together, but did seek out other friendships.
When they were in high school, they wanted their own rooms, so my parents moved them from sharing a room to each having their own. We constantly still found them sleeping in the same room by morning. Funny.
Now as a parent, my daughter is friends with twins. I stressed about if one twin would feel preferred over the other and how my daughter would make sure she didn't hurt their feelings. We do always invite the twins together for things, but mostly- I stopped stressing. They all seem quite happy, so I let them all work out the friendship bugs and they do! This is middle school, so a bit older. But just to emphasize that these things work out usually without help from us, and also to let you know how others might not be totally sure how to have their kid approach a friendship with one or both of your girls.
btw- my sisters still talk pretty much daily, but they live states apart and have well-adjusted independent lives.