A. Window -> New editor
or (slightly quicker)
B. Right-click on the tab with the code you want to see in another split view and choose “New Editor” to duplicate this tab in the same editor window (i.e. without having to open a completely new editor window as in A.)
Then drag the new tab to the right until you see an arrow, then lift your finger. Doing it this way the two views will be in sync: everything you change on the left will change on the right.