@Toni ******
How to Stop Carrying Your Old Passport
You can permanently leave your old passport at home by completing an official record transfer once you are inside Thailand.
Enter Thailand: Fly into the country using both passports (old with visa, new for stamping).
Visit Immigration: Go to a local Thai Immigration Office (such as Chaeng Wattana in Bangkok).
Get the Official Stamp: Request a "Transfer Stamp to New Passport" (Immigration Form Request).Once the officer links your DTV to your new passport in their database and stamps your new passport, your digital visa history is officially moved. From that exact moment onward, you only need to travel with your new passport.
If You Choose Not to Transfer the Stamp. If you decide not to visit a Thai Immigration office to formally transfer the records, then yes, you must carry both passports every single time you enter or exit Thailand for the entire 5-year validity of your DTV.