Wow, quite the story!
My mother is a midwife and she would encourage making out, nipple stimulation and sex as a good way to start labor, but don't think she was ever a participant.
Your story felt like the partners were really in sync with one another which I found beautiful, and though my husband(s) were supportive, I didn't experience the connection with them in such an intimate way.
Only one husband per birth ;) I've been married twice and had two children with each man.