My wife said this happens all the time & it's really not that difficult. She's had five couples she nannied for in her teenage years, & had secret sexual relationships with all five. She said the key is to read the room, then for the babysitter to make the first move, sometimes with the husband, but usually with the wife first. In her experience older wives are over sex with their hubby & don't mind. It helped that my wife has a bubbly personality & did well in beauty pageants while being every older man's wet dream as a teen.
When it's my wife and one of her female friends, I'll pump my load inside our guest - either her mouth or her cunt. It's just a matter of good manners that the guest gets the first cum.