My son is almost 22 months and I guess we have been brushing his teeth at night since he was about a year old. My son is extremely independent and has to do everything himself, which worked great for this. After his bath, while he is sitting on the counter and we are drying his hair we let him brush his own teeth, complete with toothpaste. He thinks he is so grown up. Once his hair is dry we simply tell him it is our turn. He usually hands over the toothbrush and opens wide. Maybe we are just lucky, who knows. We haven't worked our way up to twice a day yet, but some mornings he stands in the doorway and watches one of us brushing our teeth and will ask to do his, so we just hand him the toothbrush and he brushes while we do.
Good luck to you and don't worry too much about how many times a day you brush them. I commend you for brushing them at all at this age, I know several people who don't.