Conversely.... a man with one watch will never know if the watch is off but a man with two watches will at least know, not to trust either one. I have something like a dozen watches. I'm a time zone all by myself.
As far as thermometer goes, I have multiple but I only use the same one every time. For B&W work, it really doesn't matter if it's degree or two off, as long as your process is calibrated and the reading is consistent.