So the specialness of the 8•24 GC combined with a plain 8•24 is the fact that the 8•24 GC is accelerated, and will share its acceleration with other 8•24 cards via NuBus block transfer.

But, compared to later NuBus cards, the 8•24 GC's acceleration isn't that great. If you're going for best acceleration, go for a couple faster accelerated cards.

But if you're going for "cool tech, Apple-only" factor, go for an 8•24 GC+8•24. Obviously nowadays it's rather pointless, but at the time of release, it was a big selling point that you could throw in a cheaper 8•24 card and get what amounted to a second accelerated card for cheap.