My 12-bar cutter is also too short for my soap loaves. The top handle on my cutter is constructed to allow the longer piece to hang off the edge of the cutting deck, so there is no issue.Mine is a 12 bar cutter but I always cut 18 bars on it. It's actually quite easy but maybe it's just me.
I do see what @Cindy D. Is saying, that the Bud’s cutter handle doesn’t allow for this. The solution for loaves that are longer than the cutting deck is to cut off any excess length before placing the remainder of the loaf on the cutter. A Hercules or other loaf splitter is ideal for this, but a bench scraper-type of soap cutter will also work.