hi, i know that you are infatuated w/ miss 2. but, how about working w/ ur SO for say 3 months of honest to goodness communication & loyalty towards helping her feel emotionally better...less depressed, not threatened.
maybe she could benefit from an anti-depressant at a dr's suggestion & even counseling. i would be concerned about her health. if you explain this to miss 2, if she is a kind person, she will back-off for a while. if you contact her in a given amount of time, she should have no problem w/ you having some space to clarify your relationship w/ #1.
i think for her to b willing to have sex w/ you, even tho ur SO isn't keen on it...if that's what i read correctly? then, that's wrong. if i read incorreclty, sorry.
good luck. hard to combat lust.