With low water level in the rarely used second toilet, should I add water or just flush to bring water level up?

Either way will work. Might be easier to just flush and not worry about spilling water.

Hi Anxious

Flush the toilet and while in that bathroom, run water to the other fixtures (sink, tub, shower, etc.) to keep their trap seals from drying out as well.

At a minimum, you want to run the tub/shower valve and turn on the sink faucet as well as flush the toilet every couple weeks to keep the traps from drying out and to "exercise" the valves/faucets and keep the inner workings from drying out and seizing up on you down the road. In the end, this will save you money/hassles!

Mark