I'm assuming this is for an ale. If you're splitting the batch into two separate vessels that's 7.5 gallons of wort per vessel. Mr. Malty says you'd need 2.68 liters of starter, or 288 billion yeast cells, per 7.5 gal vessel @ OG of 1.055.
If it were me, I would treat each batch separately and just get two packs of liquid yeast and two 1-gallon growler bottles to make two separate 2.68 liter starters. In other words, one starter per batch of beer. Be aware you'll need to step up the starters gradually to your final volume requirement... there's plenty of info in the forums here on how to accomplish that.
Another option is to use dry yeast. You'd need approx. 1.4 packets of dry yeast per vessel if you go this route.
It may also be a good opportunity to test out different yeasts in each vessel, say Wyeast in one and White Labs (or whatever you choose) in the other.