Breeding

Question
Is it possible for the two sub species Congo African Grey and Timneh African Grey to successfully breed?

Answer
African Grey Timneh's are a hybrid offspring of this pairing.

Hybridizing is highly controversial since it waters down the species specifics genetics and could eventually eliminate the lines.  For example, a bird may look like a combination of the parents, or like one or the other of the parents.  If the bird is then sold or given away and bred based on looks, a Timneh looking parrot may have African Grey genetics and vice versa.  Though visually people might not be able to quickly tell, it's still not a pure line and never will be again.

But - to answer your question - yes, they can successfully breed.