Want to split my next batch on two yeasts and eventually serve kegged. Is it okay to store, force carb and serve a keg half full? Will i use more co2? Just not sure about the head space issue in storage and carbing...

Thanks in advance.

You will be just fine. Just be sure to purge the keg well to be sure that you removed the extra oxygen in the headspace.

Yes, you will end up using more CO2, but as long as it's purged thoroughly, you should be ok as far as oxidation, etc...