I want load two video files simultaneously, and visually compare their 'pattern', to quickly find out where to patch them (both are incomplete) together. (In audio editing, for this I would use a "multitrack editor".)
Will Premiere Elements do this?
I want to work with files like .avi and .mp4. (It would be a nice bonus if it also handles .mkv files.) I'd love to avoid recoding when possible.
It's not clear what you're trying to do, Magic.
But if you're asking if you can view two videos, side by side, in Premiere Elements, the answer is no. You can't.
You also can not mix two different formats (AVIs and MP4s, for instance) in the same project.
The program also re-encodes and re-compresses all video except for DV-AVIs, and then only when you use DV-AVI as the output format also and you don't apply any effects to your video project.
In short, while I'm not sure what you're trying to accomplish, I don't think Premiere Elements is at all the tool you're looking for to do it.
Not in Premiere Elements, Difference Matte effect in Premiere Pro might be able to help you. Add to clips on top of each other. Apply Difference Matte on top clip. Select Difference layer as Layer 1.