How to calculate an op-amp's current usage based on datasheet's supply current and its Vout terminal?
The typical current is 200uA for the package, plus whatever else you draw from connections to the op-amp output(s). You should use the maximum of 340uA rather than the typical. 

You can't stop the unused op-amp from drawing quiescent current, it does so whether you connect to the output or not, however if you connect the unused one incorrectly it might draw more current because it's oscillating or whatever. 

If you draw 1mA from the output of the used op-amp, total current could be as high as 1.34mA. 

Of course the current at the input of your boost current will have to include the voltage ratio (if you double the voltage, the current at the input will be double the output current, plus more current for the quiescent current of the switching regulator and the losses in the switching regulator.