I've no experience what-so-ever in terms of the issues you have raised...but I would like to congratulate you for starting the non-prententious version of an acceptance journey
And for having the courage to open up a discussion regarding what would be some very deep hurts and betrayals in your past.
Our parents are supposed to give us our sense of unconditional love, and sadly that does go terribly wrong too often.
It would seem a valid set of questions to be thinking about though.
And I'm sure there would be shrinks all over the world suggesting the reason you may want two would be, in part...linked to some sort of subconscious desire to have a witness present, someone who could protect you.
The fear of being alone with someone driving you to want to be with two sounds like it could just as easily be linked to an issue around unconditional love. And that the abuse happened independantly. Or there may be no links at all....hard to tell..(especially from my zero experience position)
If it's not pushing too much, are you able to explain what the fear of being alone with someone means to you/feels like/is to you ?
There are generally 2 types of drivers for our behaviours;
1. Avoiding negative events/emotions (fear leading us to run away from the lion)
2. Seeking positive events/emotions (Making sure we ring that person so we can can see them again)..
And you mention that fear of being alone with one is a driver for you. So, it sounds like you're focused on avoiding negatives emotions - I understand that...especially with abuse in your past - especially that.
Trying to avoid a situation where bad stuff may happen to you is a wise course of action. But being vulnerable is a very important part of being human, making connections, accepting love and giving love, with 1 person, or 2 or 3..
And being accepted and loved despite the fact you're vulnerable or scared or fearful....is unconditional love - return to second paragraph re lack of unconditional love from parents.
My thoughts are that unconditional love plays an equal part in mono relationships as it does in poly relationships. It's the potential issue of unconditional love not being established and valued and cherished during your childhood that stands out to me. Only an opinion though...
Have you had the opportunity to talk with someone professionally ? Such massive hurts...Sounds like you're doing loads of work - Best wishes !