Increasing the time on the stir plate is unlikely to help. Once the yeast have consumed all the nutrients they will not multiply any more, stirred or not. Increasing the size of the starter will help by giving them more to eat, so they will multiply more before running out of fuel.
I make a two-stage yeast starter. I make a 750 ml starter and let it ferment (on a stir plate) for 36 hours, then I cool it down in the refrigerator overnight. The next day I decant most of the liquid and use the slurry to make a 1.5 liter starter. The online calculator I use says that will make nearly twice as many yeast cells as just making a 1.5 liter starter in one stage. It does take two extra days, though.