Working 2 companies and extending visa question

A mate of mine is in a unique situation. He's been working for a company since last 10 years and on yearly extensions basis on employment to that. Earlier this year, he also started to work for another company and have the second company added to his work permit as well. So work for 2 companies on same work permit. Currently he's only receiving the salary from the new company and he wishes to continue his yearly extensions of stay under the new company. How does he do that? Does he have to have his current extension cancelled first or he could just use the documentations from new company to extend his yearly extension once the current extension comes to an end?