Just to be clear, I'm talking about two (or more) developers, gathering together to solve a problem collaboratively, with the end result that the code to do so

Just to be clear, I'm talking about two (or more) developers, gathering together to solve a problem collaboratively, with the end result that the code to do so is written or at least the significant questions have been answered and what remains is little more than a mechanical exercise. But I'm talking about that happening spontaneously, and not necessarily starting from a complete blank slate, being done on a specific schedule or according to any specific process or using any specific tools, or being required as formal policy.