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They're not direct sequels plot wise, meaning they simply take place in the same universe. But part 2 had cameos from part one you'll likely appreciate more after having spent 40+ hours with certain characters.
Also, this is just a personal opinion but the 2nd game is much better than the first, so it may feel like a downgrade if you go backwards. Mind you, Original Sin 1 is still an amazing game, it's just part 2 is generally considered much better.