Depends on the individuals
Well, every situation is different but if you were going to make a broad general statement it would be that it's most beneficial when metamours know and at least respect each other.
Reasoning is simple - it fosters trust and comfort. People are most afraid of what they don't know !
But there are various possible combinations where 2 (or more) metas just would not click - or even respect each other.
So you have to evaluate this potential yourself. You are the one that knows them both - their views, lifestyles etc. It's not uncommon for us to have interests in people that are leagues apart. it satisfies OUR need for variety and balance but may be a friction point with them. Or a respect point of you !
My thoughts - unless you truly believe they'd get along fabulously with or without you......
Have a conversation with BOTH of them explaining to them both that knowing them both, you feel they'd clash and it's not beneficial for anyone to have all this drama surface if it can be avoided. Good enough to just know there IS someone else - and likely always will be - and let the unique relationships develop on their own. If in the future there appears more potential for something closer between all - cross that bridge when you come to it.
My thoughts anyway....