You actually need mutliple witness accounts as well as miners.

The reason for that is because when your miner finds a block, the software switches to the witness and waits for the witness to move up in the queue. While the witness is in the queue you are not mining, but even if you do and do find a block the witness will be alrady in the queue, so you will need a second witness.

The problem with multiple witness accounts is that the reward goes to the witness account that signs a block, so rewards will be split among multiple accounts and you are not able to quickly transfer Steem Power between accounts.

Here is how you can withdraw Steem Power, it is a process taking a lot of time: https://steemit.com/guide/@cryptos/understanding-steem-power-and-how-to-power-down-your-steem