Column A or Column B required MS Access DB
I think of a scenario, where you can have a MasterUser table that will asign ID to both Customer and Prospect, auto incremented PKId can be named as UserID of the table.

let Say you create a table MasterUser and stored common details like names etc. that both Customer and Prospect have, now the unique info can be better moved to table Customer and ProspectiveCustomer.

Now as you have uniqueId for each user, no matter the user is Prospect or Customer.

So you Enquiry table will be something like


Enquiry
-----------------------------------------------
 EnquiryID  | UserID| Notes | 
       3    |    2  |   x   | 
       4    |    3  |   y   |