In my experience, that's pretty high-variance. The perfect result - one where you get anywhere from 1.5x to 10x boost of productivity - requires pairing people who communicate well, are on relatively similar skill level[0], can focus, and who both
have a good day. No distracting personal issues, no emotional problems, or other kinds of things that make you unable to focus in presence of other people. If
any of those conditions isn't met, pairing up will waste time.
There's some confounding involved, of course - for example, a good rapport with teammates can mitigate the effects of private issues. But my point is that you can't just pair people up and expect productivity to improve; it'll likely decrease, and it'll likely swing wildly from day to day.
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[0] - With respect to division of work on the task. Like e.g. a long time ago I was in a gamedev competition with a team of artists. I was nowhere near as good a programmer as they were artists, but we decided to make an art-heavy game, so our workloads balanced out and we proceeded almost in lock step.