Rhizome or tuber is technically correct but for most here it is not a big deal. You do not indicate where you are located so hopefully you are planting in a warm climate area. Assuming your supplier provided rhizomes that have at least one healthy growing point you should still see at least one of the pieces survive and grow. With a little TLC you may see both pieces produce healthy plants.
Good luck and let us know.

Kent

Thank you for the response! I inspected the pieces and it does appare that there is a growing point on one piece. Couldn't find one on the other. But I'm thinking I will plant it anyway and see what happens. I am glad to know I haven't completely killed this canna.

I'm in zone 9. Its a little too cold here right now for planting outdoors. So I figured I would get a head start and plant them in pots indoors by a sunny window and move them outside when it gets warmer.