I'm sorry for not getting back to you sooner. The key is to use inline formulas, which you can define by clicking the f(x) button at the end of each field row.
Here's an example that does what you need: http://www.mockaroo.com/dbbfc6d0
Notice how the last f(x) button is highlighted blue. If you click it you'll see the following:
if col1.nil? or col2.nil? then nil else this end
This tells mockaroo to return a blank if either col1 or col2 is blank, otherwise return the generated value.
