Well... it seems you both want these things but I'll reframe it with "willing" ok? Just to see how it reads.
- We want and are willing to stay together in a loving but non-sexual partnership. We want and are willing to be maybe lovers again? maybe coparents?
- We want and are willing to live separately.
- We want and are willing to continue to date and have cuddle sleepovers, etc.
- We want and are willing to make space for these other/new relationships but we don't want to stop seeing each other.
His partner and your partners are feeling weird now -- presumably they were ok with it when you were living together and life partners? Could ask them what changed for them. Reassure they are important to you.
- But we don't want to push people away from getting close to us. ( <--- you both don't control this willingness. That's the willingness of your other partners. You two get to state "We want and are willing to have other relationships with other people." The part that goes "we want and are willing to be close partners to YOU TWO" is what THEY get to state. Not you. )
His new sweetie is very threatened by our "partner" status. I feel like he is getting scared and scarcity feelings of- we will never find other people that will understand our connection so we should just stop calling each other life partners.
Well, could you be willing to drop the name of "life partner" and just be whatever it is you have together to make his life easier with the other partner? Or is the "life partner" name important to you and you prefer another solution?
Could he be willing to reassure his partner? Maybe page 5 & 6 could help on this jealousy article.
Or think about the green eyed monster
Meanwhile my secondary partner is triggered by me and my partners closeness and is worried I will always keep her at arms distance if he remains my life partner
Could you be willing to do same "jealousy things" above with your other partners?
What behavior do you need to demonstrate to her to show her you are not keeping her at "arm's distance?" Has she told you? Could you be willing to to that behavior if she tells you what that could be?
Have all these players talked in a group together for how to adjust to this new transition -- the living separate thing? Is that something you all want to do?
Just some ideas. Not sure what could serve you best in your situation.