Here is a guide to get started with a single system on Windows - https://steemit.com/steemit/@cryptos/getting-started-mining-steem-on-windows

Then just copy the folder with the downloaded blockchain to a second PC and edit the ini file changing the name of the miner/witness (you can use the same WIF key) and no need to register the accaunt before starting to mine it will be created when it finds a block (make sure it does not yet exist).

The problem with multiple witness/miner accoutns is that rewards get split to multiple accounts, so instead you can configure one system to be a witness/miner and all others to be just miners (don't se tthe witness username for these).