I wonder whether any large network keeps logs of channels, let alone one per server. And if they do, they probably don't have much use for a merged log except for taking up less space.

My own IRC log tool [GH: tilpner/ilc] can be used to merge logs, but I rarely use that functionality.

For two log files "a" and "b", in weechats default log format:

    ilc sort -f weechat -i <(cat a b) | ilc dedup -f weechat

I've never tested this with logs over 200MB, but sort will read the combined log into memory, which is definitely not optimal.