security assist uses the Dreamweaver Check new user name behavior for this, it has the same one column limitation.
you could do this using a recordset and the Server Validation beahviors in Form Toolkit though.
Create a recrdset that will filter the email column on the email form element, this will create a variable named:
where <recordset_name> name is the name of the recordset
create a second recordset to filter the other column on the coresponding form element.
when adding the server validation behavor, choose numeric validation, for the server variable, enter:
set the minimum number to -1 and the max to 0
add a second numeric validation for the second recordset using the <recordset_name> variable for that recordset.
this will pass validation only if those two recordsets are empty.