A very simple work around to this feature omission is to place a copy of the shot you want to refer to above the shot you are correcting and use the crop tool to partition the frame.
You can then see half of each frame. Sure it’s not ideal but it temporarily gets around the problem.
Another method is, with show detailed trimming information turned on in the preferences window, temporarily park the clip you want to compare to next to the clip you are correcting and click on the end of the clip as if you were going to carry out a trimming operation, you will then see both clips displayed in 2 monitors above the timeline.
Again not ideal but . . . .