Hi,
A guest normally has a billing address. And the billing address has by default a firstname and a lastname field.
So normally, you already have that information.
Or did you deactivate these address fields ?
Or maybe you're saying you can't use address fields ?
You can indeed also use a custom user field to gather that information. And indeed, if you do so, this field will also display on the regsitration form of HikaShop. However, it won't appear on the registration form of Joomla. So a solution would be to use the Joomla user registration page instead of HikaShop's and it wouldn't have the custom user fields.