If metronidazole 500 2x a day for BV doesn't work, would increasing the dose work?

Unlikely. Metronidazole usually works well for routine bacterial vaginosis at that dose for 7 days. If you are not responding after a few days of treatment, you may need to be evaluated for another cause of your symptoms. If you initially get better and then your symptoms return, it could be due to reinfection from your partner. Condom use will help prevent this or consider having your partner treated.
No, but...! The treatment for bacterial vaginosis is metronidazole 500 mg oral pill twice daily for 7 days. Another approach is metronidazole vaginal gel topically in the vagina once nightly for 5 days. Both approaches have been shown to be equally effective. Alternatives are clindamycin or tinidazole. If this has not worked, I would go back to your doctor for repeat testing to make sure it is BV.