The Delta 1 Water Temp II Gauge is an inline water thermometer and would be set up depending on what you wanted to measure. For example, if you want to mix hot and cold water to achieve a specific temperature then yes you would need to feed both water lines into a hot and cold water mixing unit and by regulating the flow of each feeding into the gauge measure and achieve the desired water temperature.

I can't answer directly but here's how this works in my house:My 'slop' sink in the basement has a spigot with threads. This unit threads right on--like a hose. The sink spigot is a singular piece--hot and cold is mixed into it with the hot and cold handles/faucets. Hope I described this well.

Almost....don't know why they used a cheap plastic ball valve when everything else is metal