Why, yes, you sure can. If fact, it's one of the two ways I re-use yeast and the method I employ most often. The other is to direct-pitch unwashed slurry. Why others waste time and energy washing yeast when they can simply make a slightly oversized starter and save a little off for the next slightly oversized starter is beyond me.
You mean split the yeast? I suppose you could, but you'll be reducing the amount of yeast within the initial starter and then you'd really have to step up that sample to get the appropriate amount of cells. At least that's what I think would happen.