Mr_Bo_Jangles - 1:00 am on Apr 10, 2009 (gmt 0)
It would also help for the sites not to be served by same server or server/service provider, and not look alike e.g. not same CSS features.
Also, if you opt for anon domain registration, get one from one domain provider and the other from another e.g. one from GoDaddy proxy, the other from a different provider.