How to download with youtube-dl without merging

I don't believe youtube-dl alone will do what you want. However you can combine it with a command line utility like ffmpeg.

First acquire the actual URL using youtube-dl:

youtube-dl -g "https://www.youtube.com/watch?v=V_f2QkBdbRI"

Copy the output of the command and paste it as part of the -i parameter of the next command:

ffmpeg -ss 00:00:15.00 -i "OUTPUT-OF-FIRST URL" -t 00:00:10.00 -c copy out.mp4

The -ss parameter in this position states to discard all input up until 15 seconds into the video. The -t option states to capture for 10 seconds. The rest of the command tells it to store as an mp4.

ffmpeg is a popular tool and should be in any of the popular OS repositories/package managers.

ffmpeg \
  -i input1.mp4 \
  -i input2.mp4 \
  -filter_complex '[0:v]pad=iw*2:ih[int];[int][1:v]overlay=W/2:0[vid]' \
  -map '[vid]' \
  -c:v libx264 \
  -crf 23 \
  -preset veryfast \
  output.mp4

This essentially doubles the size of input1.mp4 by padding the right side with black the same size as the original video, and then places input2.mp4 over the top of that black area with the overlay filter.

Source: https://superuser.com/questions/153160/join-videos-split-screen