How can I apply for both a volunteer visa and an extension for tourism in Thailand?

A volunteer non-O visa will get you a 90 day entry stamp into Thailand. You wouldn't need a second one if you're only staying 2 months. But volunteer visas are incredibly rare now after they were abused for so many years. I don't really know of any organizations that still offer to sponsor for them. Are you saying this conservation project has told you that they will sponsor you for a non-O volunteer visa and a work permit? Those are both required to volunteer legally.

Most places just get you to come in as a tourist and work on a tourist visa or visa exmpt entry. Many of them also give you the honor of paying for it too 😅 It's illegal but a lot of people still do it.