To calculate the total price when two medium one-topping pizzas are purchased with the incentive, we can analyze the options provided:
- The first option is "1 one-topping medium pizza for $7.99".
- The second option is "2 one-topping medium pizzas for $6.49 each".
To find the total price for two medium one-topping pizzas using the second option:
\[ Total\ Price = 2 \times 6.49 = 12.98 \]
So the total price when purchasing two medium one-topping pizzas with the incentive is $12.98.
Therefore, the correct response is $12.98.