They are frequently taken together in the titration period switching from Neurontin to Lyrica. some docs Rx it when the max of one is reached and they want to Rx more.
Basically they do the same thing, and should be taken with consideration for the max levels - combined. 1800mgs Neurontin = 300 mgs Lyrica. There is no reason I'm aware of, for taking both, together- on a long term basis.