https://medium.com/@bramcohen/double-billing-is-not-healthy-competition-b698c345b11e

Where Classic would be almost not incompatible is that everyone who owns Bitcoins would immediately own both Bitcoins and Classiccoins. This creates massive confusion in transfers: If you want to do a transfer to somebody else, are you supposed to transfer Bitcoins, or Classiccoins, or both? Some counterparties will only accept Bitcoins, either on principle or because they haven’t done the requisite development, some are going to only accept Classiccoins, because they’re partisans, and some will try to be conservative and require transfers of both. Inevitably Bitcoin and Classic will have two completely separate valuations. When stocks split their combined value doesn’t go up. You can’t create value with accounting tricks. The sum dollar value of a Bitcoin and a Classiccoin will be no greater than a Bitcoin was to begin with. That’s assuming that the chaos won’t result in an overall price drop. A plummet seems more likely.