Actually, that doesn't matter. If that were true, then multiple domain hosting wouldn't be worth the effort due to no site being able to rank in that scenario.
Yes, you have can have multiple domains on the same ip and server and link them together in a silo method.
You can even have pages within a website, ( still coming for the same ip ), link it's internal and/or subdomain affiliate pages for higher rank.
If the content is extremely good, targeted and fresh, it will only do you more good than harm. Here's how it works, as people do it all the time as " internal linking "
Say you have 3 pages in a single website, with 3 completely different but " CONTENT RELATED " products targeted for your niche. Page A is linked in a reference/suggestive statement that leads to Page B's product and the content suggestion points a link from Page B, to the product of Page C, which suggests a link to the product on Page A.
As long as you don't cause an internal blacklink effect by having them all tied together, back and forth, you will have a much better pull within the site. Even if you did cross into full backlinking, bringing them all linked together, it still wouldn't count as a negative, just would be ranked and listed as a low quiality balcklink instead of a one way, which the process above starts you out with...clean, one-way internal links...doesn't matter to the search engines where they are, onsite or offsite, as long as they are relevant and legit, they count positive to your rank.
The only reason that it would count against you for linking domains on the same server is if they aren't relevent in keyword, topic/niche and more importantly, content relation.
Hope that helps.
They both should be on separate IPs or the PR juice won't pass through. So, you should host them with different hosting companies. I use 6 different hosting companies.
