I agree with Hokerer. Yeast growth is sensitive to inoculation rates in starters. 2 vials into a 1L starter gives an inoculation rate of about 200m/ml, and you should end up with about 220b cells. That's only 10% more yeast.

1 vial in a 2L starter is 50m/ml, and will give you about 205b cells, which is 105% more yeast. MrMalty says to use 2.43L for your beer, which sounds about right.

Unless the viability of my yeast is seriously in doubt, I don't like to make starters with fewer than 100m/ml. The range of 50m/ml to 100m/ml will give you the maximum yield factor for your yeast. That's 1 vial of viable yeast in a 1-2L starter.

EDIT: Woops, forgot about your stir plate. With your stir plate, the yield factors will be much higher than the numbers I put above. 1 vial and a 1L starter will be plenty for what you're doing.