I have two tracks, one containing melody and vocals, second containing melody and vocals by a different person, can i extract just the melody?
I’ll go with “no”, but perhaps others may have some ideas.
You could try the Vocal Reduction and Isolation effect. Personally I have not used it. Also, my understanding is it currently is not working properly in 3.0.0. If you happen to have 2.4.2 on your system, you can try it there, or if you can wait a couple of days, I heard they are hoping to issue 3.0.1 Release Candidate RC01 as early as tomorrow, which should have the problem fixed.
I hope this helps. 
I have two tracks
Are they MP3? MP3 changes around and deletes some tones and sounds to get their convenient small files. So even though the backing track on the two songs seems to be the same, it’s not and doesn’t lend itself to mixing and cancellation tricks.
Koz