Epoxy
Amongst other things, I also repair boats in yards and have built a few. I also used to have a custom furniture business and repair business. So much for my experience.If your tiller has split along the grain, that is, delaminated, then if not completely apart, carefully complete the split until you have separated the pieces. Then scrape and clean off ALL of the old adhesive, mix up some epoxy and epoxy the pieces together. Use surgical tubing for the clamps, just wrap it tight so there is equal pressure all over. DO NOT use bolts, screws , dowels,biscuits, etc. they will create WEAK SPOTS and they WILL FAIL when you need the tiller the most. If you cant separate the sections, you will need a new tiller. If the split is across the grain, the tiller is toasted, and you will need a new tiller.A tiller is one of the many things on a boat that you cannot try to economize on. If delaminated or slit along the grain it is very repairable. If across the grain, add it to your firewood pile.