Late response, my browser died before posting a week ago when I was here last time. Sessions provide an intimate setting and when there is chemistry too sometimes feelings and even relations follow.

A relation can go in a thousand different directions, the personalities of the people and also chance can play a big role.

Also lasting relations change overtime and sometimes change in character. Normally I would say go for it. Your partner might be a professional mistress but if she has doubts she might refuse the session anyway.

The reason I advise against is because you talk about two mistresses. If you were ready for this you would have made up your mind which of them, and it would be only one.