From an SEO perspective you absolutely should have them on the same site.
seomoz.org/blog/subdomain…plevel-domains
I love the quote from that article… “When to use an entirely new domain… When you don’t want it to rank at the search engines.”
These days the search engines want to link to authoritative sites – sites they can trust. It is the reason why even stub articles in Wikipedia can rank for some search terms.
Yes, you may have a business reason to use separate domains (like you want to sell one of them), but there are other ways around that, like 301ing the directory when you do the sale. If you chose a different option make sure you know what you are potentially giving up by doing it…
So, from an SEO perspective – keep everything on the same site…