How to check if two form field values are unique

Hi,

I would like to know how to use a feature similar to the "check unique username" included in DW, but based on two different fields instead of just the usernamefield. For example:

• Let's say that usernumber X and name Y is already in one post of the database
• If user submits usernumber = X and name = Y the user will be forwarded to an
  errorpage
• If user submits usernumber = X and name = Z the info will be saved in the
  database
• If user submits usernumber = Q and name = Y the info will be saved in the
  database
  

Thanks,

Johan