"Crossing the stream," is an old term that was more or less invented for Ghostbusters, the movie. It's basically a product of my 'brink back the 80's' portion of my mind during toilet training.
While we were toilet training Phoenix, I would urinate first while he was watching so he could see how it was done. Then he would take his turn and urinate into the toilet, mostly. He's gotten a lot better over the year. At some point, Phoenix thought it would be a great idea for us to urinate into the toilet at the same time.
Hence, "Crossing the Streams," so we would pee at the same time and try to make our urine streams cross each other on their way to the toilet. That's all it was! Nothing complex, just crossing urine streams. So this became a pretty big habit, especially before bed time. Phoenix would look forward to it most of the time. The other times he was in trouble, and so he didn't look forward to anything, much less crossing some urine streams into the toilet.
Well one day I was a work. Phoenix announced to everyone in the entire house that he had to pee. This mostly consisted of his mommy and Sequoia. So mommy went in to watch him and make sure he didn't make a mess, which he still rarely does out of belligerence. While he was preparing his micturitional moment, he looked over at mommy and said, "Mommy don't you want to cross the streams with me?"
This of course caused some embarrassment as mommy had to explain that she couldn't.
"Why not mommy?"
"Because I don't have a tee tee. I have to sit down to pee," she explained.
A pained look came over Phoenix's face at that moment and he said, very sympathetically, "Oh mommy, I'm SOOO sorry you don't have a tee tee. That's so sad." Then he proceeded to urinate into the toilet.