I'm not sure what you mean by "half" of your dme. If you make a starter with half dme, decant and ten follow it with the other half, then you will still have the same amount of yeast cells compared to if you fermented all of your dme at once. Maybe I'm not following?
You could do a couple of things:
Mr. Malty calls for 2.36L of simple starter for a 1.090 wort. You could make a starter of 1.2L, pitch, let ferment and cool. Decant off the starter and add a FRESH 1.2L of starter, let that ferment, and then you would have your 2.4L of simple starter.
Alternatively, you could pitch onto a yeast cake from a previous batch. Those methods vary from pitching onto the cake directly or harvesting the pitch, washing and repitching the slurry.
Hope this helps.