Website too big - needs to combine 2 domains.
 

I have a website that will get too big one day for its platform.

I will need a new platform, or a second site using the same one, and connect them somehow.

What should I do about seo, if my website is too big, and I need a new site to fit more content?

Do I cross link everything? Try to find a way to 301 my new sites hompage to my original sites hompage?

How do I make it so I have 2 websites (domains), because one is filled with space, and I want them to be blended so no one knows they left one and went to the other. What do I do about google crawlers being confused, or the home page of the new site that should direct to my original homepage.

a difficult question

If you have a big site create a sitemap and notify google in Webmaster tools.