I'm a bit reluctant to share this as I have been using this 'glitch' for months now. I'd be gutted if Domino's fixes it, but I feel it must be shared. This is a means by which to essentially get Buy One Get One Free EVERYDAY online (and often cheaper than 2 for Tuesdays)
You can get 2 Medium Pizzas (any 4 toppings & crusts) for around £14.99 or 2 Large Pizzas (any 4 toppings and crusts) for around £16.99 every day of the week (although your mileage may vary)
Go into Deals on the Domino's website, and select the "Buy One Pizza & Get a Second for £3/£4/£5" option IF YOU HAVE IT.
Now instead of selecting your usual pizza. Use the "Create Your Own" option instead. You can usually choose up to 4 toppings and the crust of your choice without having to pay any extra.
This is store specific. Probably just the South East (Folkestone, Ashford, Canterbury, Dover, Deal work with varying results), but it's still worth playing about with the 'Create Your Own' option to see if it makes any of your local deals cheaper :)
EDIT: It's even £2 cheaper than the Two for Tuesday deal!!
EDIT 2: If this doesn't work for you, try the Create Your Own option on the Two for Tuesday deal, it normally works out £1.50 cheaper than selecting from the menu.