GUYZ UND3RT4L3 4ND D3LT4RUN3 C0PI3D TL0Z!11!1!!!11!1 but seriously there are some suspicious similarities between the two pairs both: - are two separate but connected games that can still both be enjoyed without knowledge of the other - have code, graphics, and characters borrowed from the previous game (but the characters are not the same people) - the second game's world is a sort of parallel universe of the first game's world
there aren't any similarities between stories, but the fact that there are this many similarities is cool, as i am a fan of the n64 zelda's and deltarune (though i have not played undertale yet)