Going Potty With Grandpa

My poor dad.
Yesterday I was making a delicious sour cream chocolate cake for my friend who just had a baby. I'd bought sour cream, but realized after I put it into the batter that I didn't have enough left in the container for the frosting. So I called my dad because he was out and about, and asked him if he could pick up some sour cream for me.
He brought it over and then Big Boy wanted to show him the big new bed. When they were downstairs, my dad asked Big Boy if he could use the bathroom. Of course, Big Boy said, "sure," and so my dad went in. Big Boy asked if he could come, too. I didn't hear my dad's response, but then I peeked down the stairs and thought I overheard Big Boy playing in his room. A few minutes later I realized they were in the bathroom together.
I called in and asked my dad if he wanted to send Big Boy out, and he said, "Well, it's too late now."
They emerged a couple seconds later.
Then, last night, when I was helping Big Boy brush his teeth, I noticed that he'd put the stool normally in front of the sink over next to the toilet. I asked him if he'd gone potty when grandpa was here and he said, "Yep, we were taking turns."