I am a managing director of a small company and hold 49% shares. I have a thai "sleeping partner" who holds 51%. I already have a work permit from this company.

Now I would like to open another company, and hold 49% shares. Can I do this?

I think not but that's only my opinion.

You might ask mod to move this question to Thai visa/work permit forum.

I think that unless the first work permit is broad enough to cover the second job, then technically a work permit would be needed for the second job. I have no idea whether that's possible or not, but otherwise someone could get a work permit as a teacher and then go run a run a real estate company. Sounds like a no-no.

Yes, you definitely can. Your WP will list your position in both companies. Don't try to open the second company without adding that to your WP or you'll be in trouble.

No problem, you can have several jobs simultaneously.

Your first employer has to agree to you having a second job, but as it is your own company that would not be a problem.

The second job and the second employer will be added into your existing WP.

You and your second employer(2nd company) will still have to qualify, i.e. enough registered capital, Thai employees etc...

And as per the above poster, you'll have to let a Thai person handle the entire registration of the second company, if the labour department notices that you have been signing stuff for the second company BEFORE holding a work permit, they can very well refuse you one!

Many lawyers will first establish the company entirely with Thai shareholders (so your name appears nowhere), apply for the WP, and when everything is done call a shareholder meeting to bring you in as a shareholder (at up to 49%)...

Yes, you definitely can. Your WP will list your position in both companies. Don't try to open the second company without adding that to your WP or you'll be in trouble.

When I made the transition from working for a mult-national to my own company we specifically made sure I got my new company on the work permit, with the existing one so that the wouldn't be any break. So yes it's possible, and in some cases important, to have two jobs/companies listed.