I don't think its as rare as you'd imagine I've met quite a few ISTp pairs in my time. I agree though for one to last for that amount of time is impressive. All this stuff sadly didn't happen two years ago, it's still happening up to a month ago. She gets into a car with me and says ('this feels like an affair') etc. It's difficult for me figure out how to find a safe psychological distance between us because - She stirs up feelings in me, I have to work with her, I genuinely want to have a good relationship with her so things go smoothly and we can help our client. But what tends to happen currently is i push her away and find it hard to deal with her then feel like i need to do small things to repair it. While she seems to playing an ethical game where she is acting quite unpredictably as well.
I was actually trying to figure out exactly what is going on as once i have a handle on it i can rationally deal with it properly. Eg - find the strength to maintain a distance where she doesn't flirt with me and its workable. I guess i thought once i could figure it out then i could more easily let my values guide me (e.g she is married stay away from her with a 50 ft pole).
There is another element of this as well - I actually kind of enjoy trying to figure it out. I probably sound like a mess but i'm its just turbulent emotions, i'm handling all this perfectly fine.