Generating a unique consecutive or random code of two letters and numbers

[Gaston_Barcia]

In an app I´m creating I need to generate a random or consecutive (it´s the same) code of two letters, and this to be the initial value on a cell. It must be unique.

The user will be able to change it, so the app must check that the code is not being used. This last thing I know how to check, but any ideas are welcome. 

To be specific... I need this kind of values:

AA, FD, Z1, X9, DG, KK, LO

Thanks