Since I want to write to the same log file from multiple Twisted
processes, I need to know if the log write is atomic.

Reading the twisted log source, I can see that the log entry is written
using only one write, so the question is if this operation if always
atomic, even for large buffers.

This depends on the semantics of the Posix write call. I'm not sure, but I think the result of multiple concurrent writes is undefined. It's doubtfull that this will work

I would probably create my own logging daemon (or use an existing logging daemon). The easy and straghtforward solution is to log to seperate files :-). As long as they are timestamped you can always merge them if any analysis is needed.