Depending on the height of your stones, and the amount of radius in your tourne, sometimes you can use the edge of the stone. Better is a couple of "slip stones" with curved surfaces of appropriate radius; you can buy them from woodworking stores. Norton makes some decent ones. A tourne being what it is, you don't need an incredibly polished edge.
Or, you can use a sharpening rod (including diamond "hones," but personally I don't care for them except for sharpening inside serrations and for emergencies in places you can't get to a stone. Camping for instance, or on a construction site.