What you need is one of those big, heavy, butcher's meat cleavers -- chopping through bone is what they are designed for.
However, though it takes more time, you'll get a nicer product in the kitchen if you use poultry shears to cut through the backbone on each side, a filet knife to cut the meat away from each side of the breastbone, and the poultry shears again to cut the breastbone free.
The backbones and breastbones, like the necks, wingtips, tails, and giblets -- along with any fat globs that you pulled out of the abdomen -- go into the stockpot.
A chicken split that way is nicer in the kitchen and on the plate because you don't have annoying bones on the edges of the meat when you're trying to bite into it and you don't have those even more annoying splinters of broken bone that you don't find until you bite down on one and drive it into your gum. Instead you get a nice, clean half-chicken ready to cook as is or process further.