Currently you can only have a single Duck Address enabled on any given browser. If you create a second Email Protection account, email will be forwarded to each, but you won’t be able to autofill and generate new Private Addresses for more than one account at a time.
To create a second account: Sign out of Email Protection (your account will continue to forward email), then go to https://duckduckgo.com/email/start to create a new account. But note that you must use a different, unique forwarding address for each account.
Prior versions on GitHub.