Can you give me some step-by-step instructions for this math problem: Mike and John dined at the Taco Palace restaurant.  They each ordered the Taco Grande Plate as their main meal, but Mike also ordered a side salad for $2, a plate of cheesy fries for $4, and a diet cola for $2.  As a result, Mike's lunch bill was twice as large as John's bill.  What was the combined total cost, in dollars, of Mike and John's lunch?