As far as I know (and I'm not an attorney) there's no hard and fast legal rule for this, but my advice is make your first business successful before you start your second; which means incorporate and register one, make it happen, and only after that worry about whether you are -- and how you are -- going to incorporate and register a second.
For the record, incorporation doesn't protect or reserve a business idea, just a name, and if you don't use that name commercially, the incorporation doesn't even protect that. Ideas aren't owned. Patents are available for inventions and copyright for creative works, but you make an idea yours by building a business, not by incorporating an entity.
Question added to topic Starting a Business • August 22, 2007
Should I incorporate several business ideas into one company or create separate companies for each idea?
I have a few online business ideas, I was wondering if it would be cost-effective to incorporate a sole company to act on behalf of each e-commerce website? Or do I need to register or incorporate each seperate business?