We advise that you open and maintain multiple Twitter accounts each related to one area of your niche. By having multiple accounts - we advise 3 to be optimum - you are able to have greater reach to a wider audience.
Each account in turn should direct followers and potential clients directly to your opt-in page.
By providing value to your followers through tweets and post content across each of your accounts, you will increase following in different areas of your niche and thus clients who are visiting your opt-in page.
You will find further information in the following training videos:
Twitter v7 > Video 3 - Opening Your Twitter Account (beginning 3 mins 30 secs)
This video from the same time point will also show you how to create multiple accounts using only 1 email address.