I generally use just a handlebar mount light. However, I recently bought a more powerful LED light (Magicshine) for my handlebar, so I moved my other light (Fenix L2D flashlight) to my helmet. The Fenix beam is pretty much flooded out by the Magicshine. It does provide some benefit as I can turn my head toward cars approaching from sidestreets, so I can be sure they see me. I also can use the helmet light to read my bike computer. Bottom line, there is no need for both lights, but the helmet light provides some minor advantages. I very much prefer the handlebar as a mount location for my primary light.