I have been advised to get two flush bolts for my double doors. I have also been advised to only order one dust excluding socket - is this correct?

. Yes this would be correct, it is recommended to get 2 flush bolts, one for the top of your door and one for the bottom. It is recommended that the flush bolt at the top of your door would be longer that the one at the bottom of your door to allow you to reach from the floor. With regard the dust excluding socket, only one of these would be required in this situation as they would only be suitable for floor fitting only.