I think google is very serious about linking, especially if the domains are in the same server and same IP. what is the best attribute to use to link your own domains. Nofollow is very hard to use as there is no point using it. How about using some different tags like rel="friend" like here http://www.w3schools.com/TAGS/att_a_rel.asp?
Thanks web_master. I haven't heard of that rel attribute but do interlink between sites on hosted on the same server. Did you find any Google resources on that topic?