First, my dentist told me not to sweat the twice a day thing. He said that if I flossed (check out the children's flossers from Dentech) and brushed once a day (before bed) I was doing better than 90% of parents with toddlers. I had been a complete nut about it, but my daughter also began a little freak-out around two years old. I was lucky, my daughter appears to have my teeth and I have never had a cavity--I DO brush my own a couple of times a day...lol.
Second, buy a couple of toothbrushes and different kinds of toothpaste and let her choose which one she is going to use that night. Don't ask her if you CAN help...avoid yes/no answers....but ask her if she wants to go first or if you are going to go first. You will set up the expectation that her teeth ARE getting brushed and you WILL have some part in it.
Third, if you aren't already....buy fluoride-free until she stops eating it. There is a lot of debate around whether it is even necessary at all, but if she drinks some tap-water every day she'll get plenty.
Fourth, don't make it a fight every night. She is testing you and it won't be the last time. If my daughter won't cooperate, I just walk out and tell her to call me when she is ready. Sometimes I discover that she is just too tired to go through the whole routine. A quick swipe a drink of water is enough.
Also, my daughter and I have made a little game. I tell her to show me her monster teeth (open wide) and heer princess teeth (smile). She even scolds me if I don't brush her tongue.
Remember that no matter how smoothly things are going, they are rapidly evolving like little cavemen...we're too old and tired to even keep up!