Wow, so many options here for different yet exciting dynamics.. it all depends on the people involved and what you each want as individuals.
I admit I'm sort of curious about why your other partner is ok with sleeping in a separate bed in your room.. is it because she enjoys watching you both? Could you incorporate that in your dynamic somehow when she's there so that you can still enjoy an intimate moment with your longer-term partner but have her present?
Poly sex can often involve a juggling of perceptions and expectations, but so long as everyone is feeling happy about it, there is no reason not to do something. It sounds like you still have a few days a week without her there, so you DO still have time to be completely private... what about including her to a limited extent (if that is what she wants and is happy with) in your intimate moments?
Just a thought (from someone who has been there).