It's not concentric, it's dual-ganged, meaning one shaft controls two wafers the same way. I can't think of too many useful uses for that in a passive guitar circuit.
Concentric pot with push-pull would be very cool. Or a push-push. But I don't think anybody makes them.
Seems to me that a ganged 250K/500K pot (or dual-concentric) could be pretty useful too.
I got mine from Mouser Electric in El Cajon, CA. They're Bournes, Mouser part number 652-PDB183GTR21504A2. The Bournes part number is PDB183-GTR21-504A2.
i just bought some pots and switches from this guy (bargainmusician2011) on eBay; super cheap. i chatted with him briefly. he said he gets them made for his company from one of the few potentiometer manufacturers. He has push-pull and standard in 500 and 250k and audio or linear taper.