On the Dominos App select the 1,2,free deal. Then create your own pizza (this only works with create your own). Select your 2 sides and pay, price will show as £12.00 like you're selecting an original cheese and tom. Have been doing this for 3 weeks now.
Edit: the same price is charged even when you choose 4 toppings, or if you do half and half 8 toppings