Creating Expense Form with either or fields

I want each line to either have mileage or an expense amount, but not both.

I have the total for the line to calculate mileage value if there is an amount in the mileage column or put in the amount spent, but I want to limit the user to only putting a value in one column or the other.

Columns in form are:

Date, Description, Miles, Amount, GL Account, Total

Miles and Account are formatted as numbers.

My total column calculation if then script works but does not prevent using both.