When thinking more. I dont really see a way how could one asset be on 2 blockchains. It seems SoG and BtV already did solve this problem at least in theory. I cant. I see unsolved double spending problem. How will Ethereun blockchain know if i will sell my asset on Counterparty blockchain?
Exactly: Ethereum's blockchain won't know that. If SoG put their tokens on more than one blockchain, you could sell a card once for each blockchain it was on, which is pointless.
Maybe if SoG were more decentralized it would be possible. For example, Factom can anchor data to multiple blockchains because it does so in an automated and distributed fashion, inserting into multiple blockchains at once. This makes it impossible for a human to interrupt the process and alter the data such that a different hash is put in the different blockchains