We will need to track the state of each button, add this script to 'General' in the movie properties.
Turn on 'Use Release' for both buttons. This enables a new event called 'On Release'. 'On Click' is called when the button is pressed. 'On Release' is called when the button is released.
Delete the current script in the 'On Click' events. Add the following for the 'On Click' and 'On Release' events of the buttons.
The state of both buttons is now being tracked. Add this script to the 'On Update' event in the movie properties. The 'On Update' event is called every time the movie updates, which can be many times a second.
When the left button, for example, is held down, the variable LeftButtonPressed is set to true. When this value is true, the X value of the circle will decrease every time 'On Update' is called.