There's no direct way to set up two separate customer accounts using the same email address, but there's two alternatives you can consider.
Set these users up as a family
Set these users up as a family
When setting up a family account you can set one user to be the "Family Manager" and then add additional users as "Family Members".
Only the family manager will need to provide an email address
The family members/dependants set up under them don't need one because family members aren't able to log into their own accounts, they are managed by the family manager
Once the family manager creates their own account and also creates the family member/dependant account, all accounts will appear to be registered to the same email address, which belongs to the family manager
All email notifications of the family member/dependant accounts will be sent to the family manager's email address
You can find more info on how to create a family member here: Adding a family member to an existing profile
And you can share this guide with customers so they can complete this set up on their own here: How can I set up and manage an account for a child or partner or any other family member
Use Gmail Plus Addressing
Use Gmail Plus Addressing
Gmail users are able to use the same Gmail email address multiple times using their "Plus Addressing" feature, also known as sub-addressing, which allows users to create variations of their email address by adding a "+" symbol and any combination of characters after their username, before the @ symbol.
Example: If George and Helen want to register for two accounts but want emails regarding their classes and payments to be sent to the same email address ([email protected]), this can be achieved by registering George to [email protected] and Helen to [email protected].
You can also use something simpler such as User+1@Gmail.com, User+2@Gmail.com, [email protected], etc.
Now George will need to log into his account using the email address [email protected], and Helen will need to log into her account using the email address [email protected].