Can I work part-time for another company in Thailand on a Non-IMM B visa with a valid work permit?

The individual with a Non-IMM B visa and a valid work permit for a Thai company is inquiring whether they can work part-time as a freelance worker for another Thai company. Two responses indicate that it's advisable to check with the labour department for clarification, as visa and work permit regulations can vary. One comment suggests that a second work permit may not be necessary provided the part-time work is in the same field as the existing work permit.