Yeah, I was confused about that at first, too. Short version is, try it and you'll see that it works. That's what I did. If you watch the log messages and various web sites, you'll start to intuit what's going on.

I don't know exactly what happens "under the hood", but at a high level it seems to go something like this:

1. Miner on some machine finds proof of work and gets added to the back of the miner-witness queue as a witness (see steemd.com/witnesses on the right side of the desktop, or the 2nd table for mobile.)
2. At that point, that particular miner acccount stops mining on all machines. If you have another account configured, and it's not already in queue, then the next one will start mining.
3. Once per minute, the witness moves up a slot in the queue.
4. When the witness gets to the front of the queue, it gets a chance to produce a block. Regardless of which machine found the proof of work, the machine running the witness produces or misses the block. As long as it's running with a good network connection, it should produce. If it's down or the network lag is high, then it won't.
5. If it produces a block, you'll see your vests increase by like 0.0004 or something in steemd.com/@[userid], and you'll see your steempower increase by 1 in your wallet at steemit.com/@[userid].
6. If it doesn't produce a block, you'll see your missed block count increase by one in steemd.com/@[userid].
7. Either way, once it comes off the queue, the miners on all machines are free to mine with that account again (depending on sort order of available accounts).

Not sure how the signaling between machines is handled. I guess it might be purely network communication or it might be storing and referencing some sort of flags in the blockchain.

The bad news is that right now, one miner is mostly dominating the queue, so it won't be easy to be looking at the screen at just the right time to see the log messages if/when you hit. You have to be lucky or patient to be watching the miner output when you happen to find proof of work.

Hope that helps.