I mean, it's an interesting idea, but I feel it'd be difficult to pull off. You would need to be careful with designs to prevent there from appearing to be two starters of the same type. (ex. two grass starters). And in needing to assure the primary type is dominant and easily identifiable in each starter, I feel the secondary types would not be able to implimented to their full potential since you don't want to essentially create two starters of the same dominant typing. How would ensure the Fire/Water starter is primarily the Fire-type starter with some water moves and not a second Water-type starter with some fire moves? I just feel it would be a waste of the secondary typing.
Additionally, the starter types have always been about creating a triangle of advantages (think rock-paper-scissors), and with your typing I just see this being quite the challenge. There would be absolutely no advantages or disadvantages type-wise as everything would be negated. The Sinnoh starters were able to pull off a double triangle with their starter typings, but there is simply no triangle of any sort with your preposed starters. Looking at your types first:
Grass/Fire takes 1x damage from Fire & Water, and 1/4 damage from Grass
Fire/Water takes 1x damage from Water & Grass, and 1/4 damage from Fire
Water/Grass takes 1x damage from Fire & Grass and 1/4 damage from Water.
Okay, so I was wrong in that there is a triangle of resistance and neutral damage. Not ideal, but I suppose it could still work.
