The solution, Gmail! Gmail has a relatively unknown feature which allows multiple address to be created on the fly.
If your email address is email@example.com for example then having the dots in the email address doesn't matter and twitter will firstname.lastname@example.org and email@example.com as different email addresses enabling you to set up different twitter accounts this way.
With Gmail you can also append text on the end of your username using the + symbol, for example any email sent to firstname.lastname@example.org will end up in the same gmail account as email@example.com, amazing!
Try it on your own Gmail account. This also works with email addresses that use Google Apps.