Thanks for the suggestion. I did some more testings.
When there was no load, at 7.5V, commanding both motors to turn forward or backward led to strange "doooooooo" sounds. Sometimes one motor turned very very slowly and the other not turning at all. Sometimes both motors did not turn.
When there was no load, at 11V, commanding both motors to move forward led to both motors moving forward. However, commanding both motors to turn backward led to one motor turning only. Strange "dooooo" sound was heard.
When there was no load, at 12V, both motors turned forward or backward as commanded.
At 12V, both motors turned as desired even with loads.
This time even at 11V with load, both motors turned as desired.
Do you know what is going on?
It seems that the system is unstable and unpredictable depending on the day I tried. How come even the recommended operation voltage for the motor is 7.2V, I need to increased the voltage to 11V to see some turning.
How come without load, I needed 12V to make both motors turned as desired. However, with load, both motors turned as desired at 11V. Also, sometimes one motor turned much faster than the other.