According to one of my friends' experiences, preparing for M.Tech along with your studies is very difficult. Often you do not get stipulated time to prepare for ESE due to the mini-projects and assignments that you get very frequently.
Also, you need to especially take care of your CGPA, but if you are not worried about the CGPA you can go ahead with preparing at the full pace like few other students who do the same.
Thus, we can conclude that preparing for ESE simultaneously is really a very big task. If you have the caliber of doing the same, you can follow your will and start accordingly.