Not the way it's been sold to me. The goal is precisely to produce code, more effectively than two devs would individually, because a concurrent second pair of

Not the way it's been sold to me. The goal is precisely to produce code, more effectively than two devs would individually, because a concurrent second pair of eyes avoids blind alleys and problems that would otherwise take a handoff to spot, and handoffs are efficiency tarpits. Knowledge transfer is an unavoidable beneficial side-effect, but it's not the point.