Diagnosing failure in dual thermostat system

What would be the symptoms of a single disc thermostat failure in a two disc system? My JD 790 hoe is running on the hot side but cools very quickly upon idling. I have checked coolant level, blew out radiators and checked temps. On immediate check when HOT, it was 210 below the thermos and 190 above. Could I have one of them failing causing low flow? I may go get 2 180's in the morning and see what happens as it was hot when I left this evening.

Diagnosing failure in dual thermostat system