Battling Forces of Friends in Love

I have a very close friend who seems to be falling in love with my ex-boyfriend, whom I've recently befriended again. He and I get along great now, three years after the horrible breakup we had. I'm now engaged to someone else and happy as a clam.

However, a week after I introduced the two, I realized they were falling for each other and I felt extremely uncomfortable. Please note, my squirming over this situation is out of jealousy, but not for him. I'm jealous because I know I would lose her friendship if they got together, just out of mere circumstances...she tends to be distant when in a relationship and this factor would only increase, given my old relationship with him.

Another reason I'm uncomfortable is that
they are practically the same person, which I find to be emotionally dangerous. They're both unsure of where they're going in life, still exploring...both fall in and out of love faster than anyone I've ever met...both depressed (one's on medication for it)...both socially awkward, paranoid and overly-sensitive...the list goes on. Knowing both of them very well, I can foretell a disaster.

I let them know of my concerns, fully aware of the consequences of my doing so. She agreed to only be his friend out of respect for me and he resisted at first but then agreed it was for the best.

Last night we hung out together and she had just developed a roll of film. He was excited to see the pictures so I naturally assumed he was in them. He was. They were very...close in those photographs. She had taken him to the airport recently and it appeared they have been hanging out quite a lot. And yes, it made me cringe a little bit.

I felt they were hiding something from me too. I'm starting to get this crazy idea that they're dating secretly. And they don't want me to feel awkward so they're hanging out with me together in hopes that my worry of 'losing their friendships' will dissipate and I'll approve. It's something I wouldn't put past them, whether the move is conscious or unconscious.

I don't want to invest too much thinking into this, as I have a relationship of my own, with its own baggage of drama that I would rather focus on. But I do want to clear my head of crazy ideas. Should I just come out and say it? Is there a cautionary way of saying 'get over each other. there's plenty of fish out there. why do you have to choose my friend/exbf?' Thanks!