It sounds to me like you have an E7 or E10 meter installed. One reading for 7Am to 7Pm and the other for 7Pm to 7Am (for instance as times could be veriable)
Although you may not have E7 or E10 (so the tarifs are both the same) It is not uncommon to install these meters for single tarif use. It saved the electric supplier having to manufacture two different meters.
Just add the two readings together to find your usage.
Yes, if it used to be E& and you are now on normal tariff just provide both readings. But I'm puzzled because when I tried to move from E7 to a single tariff I was told I'd have to pay for a replacement meter, so I stuck with E7
FF, some suppliers will transfer you to single tariff and let you keep both meters, it's easy enough to add two readings up after all. It's a matter of phoning round and finding a supplier who will do that or one that will change your meters for free.