How to stack two videos with ffmpeg if they might have different size, format, framerate etc?

Question

I have two videos in different formats, sizes, video quality, framerate etc, but I want to stack them as SIMPLE as possible. Does not need to look good, but in the final video I want to see both videos at the same time. Sound does not matter at all, just the video.

I tried to rescale the videos first and then did

ffmpeg -i hexcam_480.mp4 -i GL_480.mp4 \
 -filter_complex "hstack,format=yuv420p"\
 -c:v  libx264  -crf 18\
 outstream.mp4

But here I got a warning "More than 1000 frames duplicated" and the speed was0.00927x...

I also might need to adjust the starting time as the videos might not start at the same time.

Alternatively, anyone can recommend a simple screen recorder? Might be easier ...