The correct word in this case would be 'coworker' and NOT 'cowork.' There's no such English verb as cowork, so you'd have to use work together instead. Usually we refer to a coworker as a peer that we work(ed) with on a regular or daily basis. But we'd probably say, We worked together to decorate the stage for the performance ... if it was only a one-time occasion, or if it was with your boss. (You wouldn't dare refer to him/her as your coworker!
I wonder whether you could use collaborate even though it doesn't have the exact meaning "to work as co-workers." We say people collaborate when they work together on a specific task. (See meaning #1, where you will also find synonyms.)