Not speaking out of first hand experience (yet) but here is what mystery&co do.
1 - choose one of the girls as your main girl. You give that one more attention than the other one. So that would be the girl you f-closed few times
2 - start massaging each other. first you and the new girl massage your f-buddy. the back obviously
3 - then you and your f-buddy give a back/leg/ass massage to the new girl. You can make out over the back with your friend
4 - you take your shirt off, the two girls massage you.
After that there should be enough sexual tension around that it escalated by its own. However, it's important that YOU are the leader. They want to be lead by someone who knows what he's doing. So ... fake it till you make it
good luck