If you have 2 sites with similar content on homepage, what can you do to differentiate the 2 sites? Would creating forum or blog on the sub domains like www.domain.com/forum and www.domain.com/blog and adding unique content in the sub-domains help differentiate the 2 sites?
Yes that should help. You should also try to ensure that the homepage content isn't too similar. Just try to add unique content to each website.
Would creating forum or blog on the sub domains like domain.com/forum and domain.com/blog Actually this is a sub directory, not a sub domain. A sub domain would be like forum.domain.com or blog.domain.com. This is something you will have to setup in your zone file. If you use a sub directory with a topically relevant index page for each topic you should be fine. In my experience search engines treat sub domains as separate domains - think about sites like wordpress and blogger which allow users to setup their own domains. If each sub domain was treated as part of the www domain it would be a disaster.