2 samples of a sine wave does not result in a sawtooth wave being reproduced by the DAC. A perfect sawtooth wave actually contains infinitely high frequency content and thus can't be perfectly represented digitally.

Check out this video http://xiph.org/video/vid2.shtml which was recently posted here. Also the wikipedia page on sawtooth waves has an animation showing additive synthesis of a sawtooth wave: http://en.wikipedia.org/wiki/Sawtooth_wave