I have a low volume blog, and planning to start a forum (different folder but same domain name). So, do I need to have two site maps or one sitemap should be fine? Any suggestions for such scenarios? Cheers
I suggest to use two sitemaps. It will be easier to generate them using platform tools. For instance, if your blog is powered by WordPress and your forum by vBulletin, then you'll be able to use WordPress plugins to generate blog sitemap and vBulletin plugins to produce forum sitemap. In the other way you should create a tool that will merge both environment.
You can try the wordpress plugins sitemap by Dagon design and also add that into your robot.txt files