## Matlab, 177 bytes

Generate a binary matrix defining all allowed pairs of letters, reshape it and base-64 encode it. The base-64 encoded string (`'CR+ ... %  '`) is used as data in the program. The program reverses operations to unpack the matrix, and then reads the allowed pairs:

    x=de2bi('CR+"''        1$$ L*\"%$!! !   $!04P@<W(        0$   1"%$$100@RZP4  $$    0$ ! 1$$$$1 0  P (    $ 0 0$ ! # %  '-32)';[i,j]=find(reshape(x(1:650),26,[])');char([j i]+64)