To find the total price for two medium one-topping pizzas with the given incentive, we first look at the offer:
- The price for one medium one-topping pizza is $7.99.
- The price for two medium one-topping pizzas is $6.49 each.
Now, let's calculate the total price for the two pizzas using the second option:
\[ 2 \times 6.49 = 12.98 \]
Thus, the total price when purchasing two medium one-topping pizzas with the incentive is $12.98.
So, the correct response is:
$12.98