Construction of Sumo Bot, Suggestions required (logical) !!!?

I wouldn't link the motors together like you described. Very rarely do two motors turn at EXACTLY the same speed. More likely you will have one motor pulling the other slightly which can cause serious problems. You would be further ahead to focus on traction. Increasing the weight, increasing the surface area of wheel in contact with the ground, centering the weight over the wheels; any of this can increase the traction.