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

Click the Zoom in button (+) glyphZoom-in (up to 600%) and Zoom out button (-) glyphZoom-out (as far as 10%) buttons or
Select a scale from the % scaling drop-down arrow glyph drop-down list. You may choose:
oa specific percentage
300%, 150%, 100%, 75%, 50%, 33%, 25% or
oa 'scale-to fit' option:

Whole page

Ctrl + J.


Printed region

Ctrl + M


Printed width

Ctrl + W


Page width

Ctrl + K.


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 'ee' EEview iconEEview 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.