You can use the Other Editor Joint Scrolling command in the View menu whenever you have multiple instances of EditPad Pro running. It works whether you started them with the View|New Editor menu item or via the /newinstance command line parameter. Clicking the Other Editor Joint Scrolling item has no immediate effect other than putting a box around its icon to indicate it is active.
If you turn on Other Editor Joint Scrolling in the EditPad Pro instance that has keyboard focus, then it will broadcast a scrolling message to all other EditPad Pro instances whenever you scroll the active file. If Other Editor Joint Scrolling is turned on in any other running EditPad Pro instance, then those instances will scroll their active files to the same line whenever they receive the scrolling messages. The result is that if you turn on Other Editor Joint Scrolling in two or more EditPad Pro instances, then all those instances will scroll simultaneously. This makes it easy to compare two or more files side by side.