Robert_Charlton - 5:57 am on Jan 12, 2013 (gmt 0)

Do the sites each have independent backlinks... or are the backlinks from common sources? If the sites have a substantial number of independent inbound links, and if you're not interlinking the sites for anchor text or link juice, then you're probably OK.

If the sites don't have strong, independent link profiles, they you might consider using rel="nofollow" on the links between the sites.