Not real sure about your unit you got there. But I would think something like one is for unit shutdown refrigerant level and the other while unit has been running. To determine proper charge. Call Carrier and look in IOM.
Actually I thought it was the 19DG that had the sight glasses? My memory is fading, but...
They use two sight glasses instead of using one really large one. The level has a range that is acceptable, and the level will change with different operating conditions. As long as it's visible in either glass your fine. Also you should only use this as a guide, you still need to check approach, discharge superheat and water flows to get the full picture.
I found these sight glasses very useful, I miss them on newer machines.
I agree with Chiller Man about approach,but to me there is nothing like seeing
where level acually is. Hard to detemine sometimes with just one sightglass.
It almost all comes down to pressures and temperatures. Obtain all your Approaches, Condenser, Cooler and yes, Cooling Tower. Get all of your Delta Ps and Delta Ts. Check your Discharge SuperHeat. Always consult with your manufacturers with that one because some factor with Saturated Condensing Temperature from the Discharge Line Temperature and some factor with Saturated Discharge Temperature from the Discharge Line Temperature.