Are they dial thermometers/hygrometers, or digital? Digital will be a lot more accurate than the dials you get in a pet store will ever be.

In the hardware section at Walmart, they sell an Accurite digital thermometer and combination hygrometer. It measures two temperatures (one with a probe) so you can stick the thermometer on one side to measure those temps and the probe on the other side to keep track of that side. It also measures the humidity. It's only about 13 bucks.