for private-key you can use any key so just generate one on: https://bitaddress.org
(read step 8 in this post: https://steemit.com/steem/@pfunk/guide-maximize-your-mining-hashrate-in-windows-by-mining-steem-in-a-vm)
you can only mine one account on one machine at a time ... so you can have 2 computers mining for 2 accounts ...
also consider adding more accounts to your config for example : steemeando1, steemeando2 ... so your miner won't go idle when one account's in queue ... like this:
witness = "steemeando"
witness = "steemeando1"
witness = "steemeando2"
miner = ["steemeando","xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]
miner = ["steemeando1","xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]
miner = ["steemeando2","xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]
these accounts don't have to exist ... they'll be created when you'll find a block ... and will be accessible with the key assigned to them in the config ...