This is automatically done by OpenStudio during forward translation when you have more than one space in the same thermal zone.

It combines the Spaces into a single thermal zone, and when doing this, it converts the various potential Lights (and LightsDefinition) objects into a given lighthing level (absolute, in W, allows to sum two or more LightsDefinition that could have different lighting units).

Source:

• ForwardTranslator::translateModelPrivate calls thermalZone.combineSpaces: ForwardTranslator.cpp#L226
• In ThermalZone::combineSpaces, it calls SpaceLoad::hardsize(): ThermalZone.cpp#L1638
• For the case of Lights, you'll see that it translates you W/m2 that to a lightingLevel (W): Lights.cpp#L147