Why double-witnessing is a bad thing

From Hive account@steemed/Essential Guide to Becoming a Witness:

First WHATEVER YOU DO, DO NOT ADD YOUR WITNESS TO THE MINER!!!

Yeah, that's shouting and potentially obnoxious, but it's for your own good. If a witness tries to mint blocks on two different machines, it runs the risk of being caught for producing double work on a block, which will be interpreted as a malicious attempt to fork the chain. When this happens, the witness will likely be reported by an observer, and the totality of the witness's VESTS will be transferred to the reporter. The witness will cease to be a witness and it will be a dark day for the witness's owner.

You have been warned.