No Idea. When the septic tank was removed this was the point where the connection was made to the house.
My previous home had sewers with cleanouts like that every 30ft or so. But only a single pipe and cap - not doubled up.
At the farm, we have a septic tank so no more cleanouts. Just throw a ripe flat critter in there once a year to activate it. Don't even need to pump it out.

