I didn't know about body doubling so in the mean time let me ask you some things:
- do you consider the friendly competition aspect (seeing someone do a task revives inspiration, desire, confidence, motivation to do it, when the other is tired you feel happy to fill the role)
- the emotional bond from sharing: sharing any task is often a strong bond (even just carrying a load), pain and success
- the intellectual benefits: a bit like the first point but searching for solutions gets much much cheaper with a peer. You can validate point of view / theories, feel less emotional pain from doubts it the other doesn't know too
I had more topics but I forgot