I take it from your comments that you are limited to your 2 liter flask for a starter size. In this case, you can step up your starter with a 2-liter starter by cold crashing the first starter, decant the clear wort from the top and then adding in another 2 liters or so of starter wort.
My actual practice is to make the first starter, cold crash and decant, and then pour half of the remaining slurry into a sanitized jar and making a starter with the other half. For very large amounts of yeast, I will split the first starter into two flasks and add starter wort to both of them.