Are you sure they are actually null and not a string with the word "Null"?
If they are actually null, create a calculated field called CombinedDim with the following formula IFNULL([Dimension 1],[Dimension 2]).
If they just contain the word null, I'd create a calculated field as follows IF [Dimension 1] = "Null" then [Dimension 2] else [Dimension 1] END
I switched them from true nulls to string nulls. Thanks, both approaches worked.