Bombs packed with ball bearings, which are designed to do maximum damage to people. Perhaps designed to embarrass Thailand who are currently hosting the ASEAN security meeting, with Mike Pompeo making speeches.
If they are both legal passports, you would appear to be free to use either when you arrive, but have to use the same one going out. If you use the other passport next time you come in, why should there be a problem?
No benefit in extending @ 1,900 Baht a time until the final entry. Make sure you come back on the final entry the day before the visa expires for 60 days stamp, and then you can extend that for 30 days. That way you actually get 9 months in Thailand on your 6-month visa.
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