Nishan means a sign. While its derivative, Nishani is a 'distinctive sign' or identification, or a souvenir or memorial.
Here in the song of Jhootha hi Sahi, Nishani would mean a souvenir, and hence, Do Nishaniyan would mean two souvenirs.
Update: Unlike I had expected, the song uses the meaning of nishani as not just an identification mark, but an identity itself. Not a regular meaning of the word, but quite expresses the thought here. For detailed meaning, check the post on song.