Take fair warning from others' replies that it may be an uphill struggle for you if your potential employer is so completely clueless about employing a foreigner. The idea that they sent you to your own embassy to ask about working in Thailand is embarrassing.
- Brilliant, Monsieur Poirot, but I'm capable of remembering which passport I'm travelling on. Entry stamps are a great help with that 🤨. All passports used are in the system, and they are free to ask to see the other one/s.
Be aware.. they are aware of your other passport/s. I also have more than one passport and I have often been asked to show others on entry (in Thailand and several other Asian countries).
As a general note, in a country with so many visa types, entry types, and exemptions, play your part and physically show the IO what you want to enter on. Sometimes people have a visa in their passport and don't want to use it yet. Others will be assumed exempt unless shown otherwise. Eliminate the guessing games - show your visa.
Many accommodation providers will not supply a TM30. If you are checking into a new place just before you apply for the extension/s, ask first whether they can give you a TM30 receipt or not.