Hi. All the data that I have from commercial manufacturers related to chiller performance with two compressors reflects reflect the performance of the unit as a whole. My opinion is that you have two options:

1. Look for the manufacturer data to check if you can build energy consumption, efficiency and capacity curves which reflects the performance of the chiller as a single unit with two compressors.
2. Model two chillers in series with curves for each one. Curves can be equal or different, depending on manufacturer data.