Can I stay in Thailand for 6 months by applying for a Non-O visa twice?

Before your Visa is running out you can go to any Immigration Office and extended for another month up to one year! This information I got from the website of Thai embassy Berlin

That is a little more complicated than it sounds. That's because you would need to leave Thailand after the initial non-O entry was running out, and if you wanted another non-O you would need to apply from a nearby embassy or consulate.

I assume your passport qualifies for visa exempt, so a better plan is probably to apply for the non-O before you leave for Thailand. Then after 90 days do a border bounce and return visa exempt (no visa) and then you'll receive a 60 day stamp. Apply for the 30-day extension from immigration, which will give you a total of 180 days, same as 2 non-O visas but will not require the extra non-O visa application.