They should definitely start hanging out together if they don't know each other. that is where to start! I thought they knew each other too. Yup, would invite the ex over for a supper that you would make and allow them to talk and get to know each other while you treat them like the queens they are. Lavish some good hosting on them and make them feel comradeship with each other right away. Make it fun and light, where a cute apron, whatever you can think of to make it a fun evening of laughing and lightness.
My fear here is that GF will fall into the trap of motivational butt kicker for the two of you (ex and you).... not fair. I fall into that quite often. If and when this all works out, make sure that you keep your relationship with both separate as no metamour should be butt kicking a relationship that isn't hers... your relationship with the GF is a separate thing entirely from the friendship you have with the ex. Getting together and having a poly relationship, will not fix the relationship you had before, that didn't work out. You may all get along, but that is it. It's up to you and the ex to work out what happened that you all didn't work the first time round.
I'm also concerned that you are hesitating on the thought of either of them having other lovers. If you are finding your back is getting up over that one, then it might just well mean that ex stay a really good friend and not take it anywhere else... unless you can commit to the change that will occur, don't do it... your ladies have minds of their own and will not have your undivided attention all on them all of the time any more. They will want something more in some way to make up for that, and the first thing they will think about is finding other lovers. I can pretty much guarantee that they will have no problem once they get out there. Women generally don't, it's the guys that have a hard time finding another lover. It seems this is the very first step, as is the one noted above.
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