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I would agree focus on one twitter account. The only reason you might want a second twitter account is from something like customer service. However very often you will find that this just leads to confusion and customers will mention your marketing twitter account rather than your customer service twitter account. As for the tone of the account that is something that you have to work out on your own. You tone on twitter should reflect your brand. But remember it is a social medium and so generally the tone is more conversational. Hope that this helps.