can people work 2 jobs in Japan?

I have a valid work visa with my current company. But now, I see there is another job draws my interest. I am considering to keep my current job for the sponsorship (work less hours) and work on the other job. In this case, is it possible to have 2 jobs in Japan?

Yes, as long as your new job is similar to your current job- you need special permission to do work that is not covered in the permitted job description (designated activities) for your current visa.

If you will no longer be fulltime at your main job, you also need to make sure you will be okay at vise extension time- you may need documents from both companies to support your application, so make sure they are prepared to give them to you in that situation.