We have multiple accounts (http://usgs.gov/socialmedia). There’s a benefit to having multiple Twitter accounts in that you can follow specific information you find relevant. From time to time, we will RT certain tweets from our other accounts that could be relevant to wider audience but may have originated with a smaller, niche account.
However, if you’re going to allow multiple Twitter accounts, then you need to give set up a registration process that will include questions about their communications strategy–what are your goals, how will Twitter help you meet those goals, is Twitter even the right tool to use, etc. Some people that don’t think in terms of longer-term strategy or communications planning may not understand the consequences of setting up an account without a strategy. The registration process is a chance for you to present those questions to them and get their minds working. Those that are serious about having an account and have a good idea of why they want to have one will contact you and ask for your help/guidance. Then you can educate them on the policies you have in place, best practices, brainstorm ideas on how to manage the account, etc. It’s also an opportunity to compare their account idea to existing accounts to find duplication before it happens.