Durock vs hardibacker
There are a few options for cutting cement backer board like HardiBacker500 (1/2")
PATHETIC: Utility Knife.
GOOD: Carbide Tipped Scoring tool, not bad for straight line on 1/4" Hardibacker, but a waste of time for use on 1/2".
BETTER: Circular Saw with HardiBacker Blade (used outside only). I've been doing it this way for years, but it's a dusty mess, unhealthy, etc.
BEST: DeWalt makes a cement board electric cutting tool. Model D28605 for $269.00 at of all places drugstore.com. No Tax, No Shipping cost.
I just ordered a DeWalt D28605 after nearly 2 years after a contractor friend showing me his electric shears cutting HardiBacker for a shower. After much searching on Amazon.com, HomeDepot.com, etc. It was Drugstore.com that had the 1/2" version. I can tell you now I paid extra close attention to the size after I accidentally purchase some shears that were only meant for sheet metal.
There is a readily available 1/4" version of the DeWalt Electric Cement Board Shear for $220-$230, but then your limiting yourself to only 5/16" capacity to do HardiSiding and 1/4" Hardibacker and will never be able to do 1/2" boards. Your choice.