If they are in the same table, you can quite simply concatenate them in a query:
Field - FullName: [Forename] & " " & [Surname]
or
Field - FullName: [Surname] & ", " & [Forename]
It all depends how you'd wish to display them which of the options above you want.
The first will return a calculated field in your query as John Smith, the second will return Smith, John.