the Bulb/diode will draw what it wants off the battery.. double the load is halving the run time .

I Made Up A 5 D cell [1.2v per cell rechargeable] and wired a Headlight .. Old Union reflector

for flange base with Halogen Bulbs I got at radio shack.

and a regular 0.6w bulb in a on the rear Mudguard tail light for a 12 Mile commut I had thru farm country

circa 1990.. turned it of by unplugging the wire to the batteries ..