To change the scale at which both files are shown you may:

Click the Zoom-in and Zoom-out buttons or
Select a scale option from the drop-down list: 300%, 150%, 100%, 75%, 50%, 33%, 25%.

If you have opted to show the EscapeE toolbar, you may also change the scale at which each individual file is shown. The scale value shown in the EEview Control panel is not changed. If you then scale or zoom using the EEview tools it is this value that is changed and both files are rescaled to the same value.