I have a really hard time, personally, with "rules" and "allowing" in poly. No offence to Derrythe on this one... as you say, different types of poly. I guess I have just been doing this way too long to be okay with putting any kind of box around things and setting everything up as if it is going to fail before it even happens. I guess rules might be built out of fear. I guess I just don't have any any more. There is trust there now.
I think if I were in your situation I would get on having an big 'ol meet and greet and see where it goes. You can have a coffee, or a beer or go for supper and everyone can get to know each other. They might be very different, but they are grown ups and we all have to get along with people that are not like us. You never know right? Maybe they will. They have like friends after all.
By meeting you can ask them both face to face together what would make them feel comfortable when you all happen to meet in social gathering. You can also ask them both how they would like to see time divided up... doing all of that together is the best way to ensure that they don't get some fantastical idea about who the other is and start bad mouthing or feeling resentful. They should have each others info I think, in case they have their own stuff to resolve on that kind of issue.
After that they don't have to hang out all the time, but being civil and courteous is appropriate and responsible. If something comes up they can call on each other, or you can all get together again. You might even schedules a meet for a few months from now and treat them both to dinner or something as a way to bring about a positive appreciative air... isn't that what this is all about, harmony and loving each other. It is to me anyway...
Rules tend to take away from the natural flow and positive community building I have found. There is enough rules we are suppose to follow. Poly for me is about making my own way with others that want to share in that experience with me and make their way also... not about telling others what to do and setting things up without any breathing room. Bedsides, mutual respect in metamour relationships make a HUGE difference to the longevity, grounding relationships have... all of them together.
Healthy tribes make healthy people I think. We are all connected and you can start that positively by acting and being positive and believing in their ability to be civil to one another. You never know how many other ways everyones lives will cross paths. Poly communities tend to cross over pretty quick in my experience, they will likely meet in more ways than one.
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