What's the trick to cutting concrete blocks in half?

I am blocking up some old foundation windows using 8 x 8 x 16 concrete
blocks and I need some partial (half) blocks to complete the run. I
have a masonry blade, a 3-inch stone chisel and a 3-lb hammer. I've
been scoring the block in the middle, then using the chisel to crack
it.

I can't get the block to split cleanly in the middle -- one end ends up
crumbled. I do get 1 decent half block for my effort, but waste the
other half.

Can this be done?