DDJ SB3

Hi, new to DDJ SB3, 

I am a vinyl DJ and previously used Serato Scratch Live up until my Macbook Pro quit on me. In other words, I know how to mix. I haven't DJ since 2008

I have the first track ( 126 bpm ) playing on the speaker. I now have the second track, different BPM (128 bpm ) listening to it on the headphone ( monitoring ) and have it matching track one ( on beat using the TEMPO slider). I now am mixing it by using the Mixer slider from Track One to Track Two. As I do this, Track Two is now playing through the speaker along with Track One and the beats are not matching, I hear the double beats clashing. I do not use sync (for beginners

Am I doing something wrong? Is this common with this controller? Or is this controller defected? Date purchased, March 5, 2020

Tony Ramos