Just as @Ben Richards as mentioned you should right-click the tab of the file and click "clone to other view".
Initially this would be vertical, you can rotate or "flip" it horizontal by right clicking the dotted line and choose then click "Rotate to the left" or "Rotate to the right"
A point of note it that you can do this for multiple windows
bug a bug i found is that if you originally have multiple tabs open, say A B C. and you cloned A say to AA, if you click tab B or C, AA would still be shown for all of the tabs without a clone. I find this inconvenient.
If you however clone B and or C, you will also have multiple tabs in the cloned windows
![notepad++ multiple tabs](../../I/static/images/42fe23140a9a8c225a5c39378fcdca21a1130b68622b4313d1ffd522a5d9728a.jpg)
4
possible duplicate of How can I view two files side by side in Notepad++
– phuclv – 2015-07-29T06:31:28.267