Nope, a 2*18650 flashlight won’t help you for two reasons:

  • At some point, you will remove one of the batteries of your flashlight to use it with your headlamp. Where are you going to store the depleted battery? Inside your flashlight. So, from now on, every time you use your flashlight it can blow up… Even worse, if it has an electronic switch (always standby) it can blow up at any moment!
  • If you are going to use 2*any lithium batteries (connected in series or parallel), they need to be matched. Same brand and model, same purchase date, same charging cycles, same mild/hard use etc. Use only one of them a few times and the pair isn’t matched anymore

In my opinion, buy a small battery holder for a 18650 or two and get whichever flashlight you like. Go wild! Get a Lumintop ODL20C or a Thrunite Cat v6