Reaper most likely won't get a second video window soon as it's been stated by the devs that Reaper is not to become a full featured video editor, but continue to be developed as an audio workstation software instead.
Sounds like you need a source monitor and perhaps should consider trying some of the video editors out that.
Shotcut is pretty decent if it can read your video files, and it's free.
http://shotcut.org/bin/view/Shotcut
Lightworks has a good free version as well.
https://www.lwks.com/
Inexpensive and with a very decent amount of features is Sony Movie Studio.
And Adobe Premiere costs $20 a month at most. A trial is available as well. It can read most things.
For all these video editors you can check out videos on Youtube and elsewhere to get an overview if this is a video editor for your needs.