Use natural diamond to grind carbide and CBN for high-speed steel.
A diamond wheel may seem expensive, but if you reserve it only for carbide tools, and don't abuse the thing, but let it cut at its own rate, it will last a long long time. Compare this to sharpening carbides on a green silicon carbide wheel: yucky! Green wheels require frequent dressing to keep them flat, and they have more of a tendency to chip the edge of the nice tool you are trying to form. Maybe use a green stone with those brazed on inserts where you've got to grind steel and carbide together at the same time. Grind a little extra front clearance using the green wheel, and then finish the carbide edge on the diamond wheel, giving a little less clearance so you can keep the steel body of the toolbit away from the diamond.