If I work for two different companies, do I need two certificates?

The short answer is yes.

The long answer is that generally speaking, your tasks from one job to another will be different, which means the hands-on practice and evaluation will be different.

Also, your employer needs to file the records of your training which are specific to your tasks in that company, so they will not risk sanctions imposed by the Regulators just based on your promises that you are trained.