Ok, this is a bit of a strange one and possibly a misprice.
The deal is any two 9" pizzas for £3.30. To find it go to just eat and find the margo menu, then go to special offers and select menu item number 80. The double pizza 1 deal. You then select the 2 pizzas and order.
I say it may be a misprice because number 81. Is for 2 x 12" pizzas and this is £15... Also there is a 3 x 9" pizza deal, also £15..
There is also a £0.40 card charge and possible delivery charges but still IMO a great offer.