You could but then both motors would move forwards and backwards equally, you would have no steering and be unable to turn the vehicle.
Each stick operates 2 channels, one by up/down, the other by left/right.
Reading through the manual for your transmitter again, it is capable of switching between Mode 1 & 2 but not capable of channel mixing which would allow 1-stick control of movement.
Rather than one stick for hull forward/backwards and turn, the other stick for turret traverse (left/right), gun elevation (up/down), you are limited to controlling movement of the tank hull using both sticks up/down operating the drive motors independently and one stick left/right to do the turret traverse whilst other stick left/right moves gun up & down.