I believe the root of jealousy is insecurity. If I am secure in my relationship/s (whether mono OR poly) then there would be no reason for jealousy.

Thus:
Now, from what I understand I assume Couple 1 have a somewhat open relationship whereas Couple 2 is very strictly monogamous.
I'm not sure if one can simply make such an assumption.

My 'read' on the described situation is rather:
The man from couple 1 is secure in the place he occupies in his partners life.
The man in couple 2 imo feels threatened. He is insecure about the place he occupies in her life. If he wasn't worried he wouldn't be jealous and most certainly be sulking.

My Doms other girl is my best friend. I Love knowing they are spending time together just as she feels the same way when He is spending time with me.
Why don't I feel jealous? Because I am secure in knowing the role I play in His life.
When I feel a slight twinge....I recognise it as a sign of insecurity and I make sure we address it. And things settle again.

Now, I'm certainly not suggesting All monogamous people who prefer not to share are inherently insecure in their relationships, but rather that jealousy is a flashing light (in mono and poly) of a deeper problem.