You're asking for a guess based on pretty limited information. Troubleshooting is a lot easier done with a meter, than relying on speculation.
How long between one light failing and then the other? You 'believe' the bulbs were replaced - were actually replaced? If so, did that fix the problem temporarily or have no effect?
Switched power goes to the headlight ignitors and from there to the bulbs. You've got a bad switch, bad wiring, bad connections, bad ignitors, or bad bulbs. If you've got power to the ignitors, and the bulbs were replaced without effect, that gives you a good starting point. Someone needs to check the circuit - carefully, as they operate at extremely high voltages.