Hi, I have a 15 day stamp from Mae Sai border in my old passport valid until December 10, and now I have a new passport.

I wonder if I can just go to the border with my 2 passports on that day, and will they start stamping in my new one?

Thanks, Ray

You should go to immigration office to have it done unless they tell you otherwise. At border they may do but could tell you to return to immigration first as you are not legal in Thailand until it is done. There needs to be a transfer entry made in the new passport.