While I'm not a 605 "guru" as you pointed, I'll have a go at it and someone else might correct if I'm wrong.
Since you need both loops to detect the overheat condition so they trigger the alarms, by grounding the opposite loop of what you selected with the knob (grounding it puts the system out of operation, i.e. breaking the circuit) you test two things: One, that the grounding is working correctly for one loop and two, you're testing the proper normal operation of the other loop. Why is the grounding necessary? If the other loop doesn't ground, it will trigger the corresponding alarm since both loops would "detect" the overheat condition in the testing.
You need to ground one loop so you can test either loop individually, otherwise you'll get the warning light/bell/whatever since they are both "detecting" an overheat condition.
Hope it helps somehow! Spanish being my native language and talking about an aircraft I have absolutely no clue may not help in my redaction, but hey...!