I've often made 1.5 liter starters with 1 1/2 - 2 cups DME and split between 2 batches with no trouble at all. But, in my case the starter is sitting on a stir plate for up to a day and is plenty active before the pitch, so YMMV.
Another option is to brew the first batch and wait until you're ready to transfer it (to secondary/keg) before starting the second. Then, brew the second and siphon onto the yeast cake left behind by the first batch (note - if beer styles differ, lighter/less hoppier beer should be first). I did this for the first time tonight and the fermentation was fully active within about 3 hours.
Atomic Dog Brewery
On Deck: Centennial IPA, Dry Irish Stout
P: Cascade Pale Ale
T: Carbed Crystal Light, Root Beer, Vanilla Cream Soda, Tripel IPA, Apfelwein
e^(pi*i) + 1 = 0