To be clear - I didn't mean that the two _roles_ are mutually exclusive. The opposite - I think that definitely, there are common quality required between software engineering and technical writing.
What I meant is that in real world, due to times constraints, they are. As a sibling poster commented, working as a hobby ends up being a work in itself, so doing both things ends up being, effectively, overtime.
And as a consequence, all the general considerations about overtime apply.
It's a fascinating experience. How being a good writer and software engineer go hand in hand. If my library is not easy to understand, my book won't be succesful. I'm constantly going back to edit the library to make sure the book / related concepts can be grokked.
I feel this is a fairly general truism. Great software engineers are often technical writers. Both involve understanding what the 'reader' expects to see, needs to understand, and how the whole narrative/program fits together cohesively. I'd encourage anyone who wants to up their software engineering game to work on their writing skills first.