If you want to merge two logs, you can use an included script to merge the logs, which can also do DNS resolution. This external script doesn't use threads/forked processes either.
But... awstats itself can skip records it has seen before. This feature is fairly useless because it can only work well if you don't need to look up DNS or merge logs. You end up doing the hard work multiple times if you want multiple runs of awstats.
I would like to run awstats multiple times a day, just to see the numbers update. Basically, I can't because DNS resolution takes too long.