It depends on the cultivar being grown.......some grow much larger than others. But in general, 2 plants in a single whiskey barrel is crowding things a bit. In fact, a 10G container is about as small as you should go.

thanks for your reply gardengal. they are southern highbush blueberries: one is called jewel the other sunshine blue. This will be my first time growing them. They obviously really need a lot of space!

Sunshine Blue is a little unique in that it is a semi-dwarf evergreen variety bred specifically for small gardens and container culture. But it still gets to be a pretty decent size.......3-4 feet by 3-4 feet. It will fill a whiskey barrel all on its own. Jewel gets much bigger.