You can use a tile saw & rotate the rock a few times to get a through cut. Or you can buy a small cheap band saw and put a diamond edge saw blade in it as used for cutting frags. You can even buy a diamond edged blade for a hack saw. These things cut live rock fairly quickly. And yes, as said, a local stone cutter (that cuts stone counter tops) can do it for you, but that may get your live rock contaminated with something that was in the other stones like heavy metal vanes.