Using one email address for multiple Practice Ignition accounts is currently not supported although this is something that we're looking into.
If you're using Google as your email provider, you can take advantage of their feature that allows you to create an alias for your email address. This will seem like your using multiple email addresses but in reality, you're only using one.
To do this, simply add +(desiredaliashere) between your real user name and the domain name. e.g. [email protected]
Read more about Google alias here.