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Google is a registrar and will easily be able to tell that you own both sites.
I really should put the sites on different hosts (for uptime reasons), but I don't really feel like changing name registrars.
But I probably should....
You used singular and plural forms as an example; if the intent is simply to catch different forms of the same keyword you might better spend time fully developing one site -- with semantically rich pages, of course -- and pointing the unused domain at it to catch typo traffic.