hi.
met a nice young girl went for a date, started going out etc.
she’s leaving town soon, in september this year. can’t be helped.
i kinda like her gitlfriend, and i think she’s persuadable.
if you’ve had experience of this please tell me the most cautious approach to achieve success, plus what mones suppliment will aid tis situation.
I have no prior experience or knowledge at this, so this is just my own suggestion.
You should set up a chill out with all of you three before the girl leaves. Act as you normally would towards her girlfriend but drop VERY subtle hints. You should make it so casual that even your girl wouldn’t be offended. Just make sure to communicate to BOTH of them that they should keep in touch.
Pheromones wise, go with AMMO, IS and IG.
After that you can work it up from there by asking her girlfriend out, after a week or so, just for a chillout because you miss the old days that you all used to hangout. If you were to meet her up, then IG (with or without AMMO, your choice) + IS + IH.
Hope that helped a bit 
EDIT: As an afterthought, you should be straightforward and make it clear that your relationship between you and the girl you are seeing will be “platonic” only since she’s leaving. However, it will be hard getting back to her if you will later. It’s the best way to be clear, straightforward and honest so no hearts are broken.
oh kay.
I like, the friend approach, give me an excuse to call her and hang out with her after current girlfriend has left for good.
i think i’ve dropped suttle hints about being interested, and i think she has returned the interest. assuming that i haven’t made this up in my head, i need to find a way of easing naturally into the desired situation; just like you suggest.