We don’t really mine the ETH coin. We mine the blocks on the Ethereum blockchain and get ETH as a reward for the work. “Mining the block” is needed to confirm the transactions contained within that particular block.

Now with PoS the confirmation process will be different, using the stakeholders to “mint” new blocks rather than using the hashrate workers to “mine” new blocks. Therefore everything that happens on the blockchain will be confirmed by the PoS system. I don’t see how it could co-exist with PoW on the same Ethereum mainnet.

Again this is only my understanding.