Well in my darkroom I use the little Ilford thermometer, it's nice and small, comes with it's own little holder. I'm on my second one as after about 25 years, the etchings semed to have faded, so I bought my new one about 7 years ago.
I also have a very expensive, and supposedly very accurate digital thermometer. It's an Anritherm HL600 type K unit. It's range is from -200~1200ºC.
When I had some colour processing problems I dragged this unit to the darkroom and did some checking. However it's overkill for the darkroom and has one getting all concerned about whether or not it's too hot or too cold.
Ole is correct, just use one thermometer. Didn't confucius say, "Man who has two clocks, never know correct time".
Mick.