For
SEO purposes, it might be better to buy another IP for that domain as it will make it look like as if the site is hosted on a different server, since it will have a different IP. If the IPs are shared, the site will still work but if you link to one site from the other, it probably won't benefit much as the search engine will pick up that link as if it's coming from the same site due to the shared IP address. Or at least that's what I've heard.