How to calculate two different text fields if given value to automatically populate to a third field

Hi, I have created a calculating document.  I am having trouble with getting two different text fields to automatically add value to a third text field if they have value put into them by the user.  Text field one is called FirstBalance, text field two is called SecondBalance, and the third text field is called RecordFee.  I am trying to calculate a $45 fee to the text field RecordFee if the user of the document inserts a number into the text field FirstBalance and an additional $45 if the user inserts a number into the Second Balance for a total of $90 if both text fields have value, but I only want the $45 populate if text field FirstBalance is the only one the user has inserted a number into.  By an example I found I can get the first $45 to populate, but can not get the $90 to populate if the text field SecondBalance has a number inserted.  Can someone help???