Here's a quick run-down of sword training videos.
The two trickiest parts of swordplay are knowing what you are doing wrong and using it on someone
Neither of these are possible alone.
If the grip slips in your hand when you swing, you are as effective a swordsman as your average movie star. However, you can only look at your hand before and after the swing, not during. You also cannot see whether you are leaning, your head is bobbing, or your tip is wobbling. Trust me, you can be in front of a mirror checking everything, believing that you are perfect, and your instructor comes along and corrects every single thing.
As for pairs practice, impossible for obvious reasons. Don't try arming your roommate for this sort of thing, it doesn't work (trust me on this).
This is of course, working under the bad assumption that the video is of high quality. Knowing the above two paragraphs, most masters avoid video like the plague, and those that are done are largely done as supplimental work (note: big generalization here). You don't want to train in the kenjitsu equivalent of Tae-Bo.
Fencing Club at UH