Do i need a third opinion if one dentist tells me i have 2 tiny cavities that need to be filled and another dentist tells me that one of those 2 tiny cavities is just a stain on the tooth, while the other cavity might need work due to an old filling?

You can get a third opinion but don't be surprised if you get a completely different opinion. Each practitioner may diagnose you differently. How about saving a lot of time and restore the one tooth that needs a restoration and watch the other one every six months?

If the tooth with a stain comes off by cleaning it, than no cavity is present. Many old fillings wear out, so have a new filling placed. Anytime I see a possible cavity or stain, I try to scrape it off, if it doesn't come off, I probe tooth to see if it has soft enamel and or dentin. An x-ray is also helpful. If alleged stain is still there, than a filling is indicated.