Multiple SIMs on 1 Account

Hi

I already have a smarty sim and account.  I wish to add an additional SIM to my account for my child.  However from what I'm seeing on the website/app - it suggests I need to create a new account for the 2nd SIM card.  I can't open a second account because my email address is already being used for my account.

Anyone come across a similar problem?

Thanks

Susan