Club house seating is when one man sits on the toilet tank dropping an upper decker while another man sits on the bowl backwards while giving the man on the tank a BJ.
A phrase used for the act of relieving oneself while sitting on the spread apart lap of another individual while they, too, are relieving themselves on the toilet. The latter individual is then considered the 'toilet seat'. This is typically done in the woman on top position.
Trucker 1 "Hey Boomer, I'm 10-8 to the Windy City, I fancy slip seating Mrs Boomer, 42?"
Trucker 2 "That's a big 10-4 Admiral Scuttlebutt. I'll be checking out the Junk Buzzard of a lot lizard in Choo-Choo town"