It seems that in each of these parties there is a leading couple who initiates things. When Clara & Stas and Cleo and Axel are together, Clara & and (mainly) Stas are the leaders and initiates things. When Cleo and Axel and Agnes and Carl are together, it's Axel and (mainly) Cleo. And the "non-leading" couple is a bit less comfortable in the beginning.
I'd say that they are all ready to do and experiment lots of things but none of them is willing to share for full sex. Maybe next time they'll be joined byAgnes and Carl , for other games but I'd be surprised if it came to swapping. But after all, they are the only ones who know...