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Stuart, I think this will depend on the context you are using when you say campaign? Assuming you are using a Twitter button and are wanting to get social metrics from it for the site overall, I would think you are better off with only one site wide. Also, of the plugins available, I am not aware of any that give you multiple options. That said, if you want to use your personal for a page and your business for a page (assuming you are not blogging where you could set the personal as an example to the blog portion and the business to the site pages portion) you are likely able to use two different plugins one for pages where in you are using personal and one for business.  On plugins that are site wide, you will likely need to have a blog for one and the site for the other. Hope this helps,