Combine audio and video file without creating a 3rd file if this is the wrong place to post this let me know.

I want to download a very large video (9.1GiB) from youtube on my device and I have ~10GB free storage space for it.
Because it's youtube, I have to download video and audio separately and combine them afterwards. I'd normally do that using ffmpeg. I'd have to let it create the combined video in a new ~9.1GiB file because it can't overwrite it's input. The video only file of similar size must exist during the whole process.
Problem is: I can't store 2 9.1GiB files, only one.        

The possible solutions would be to edit the video only file to have an audio stream or have a super intelligent buffer system that'd delete the parts of the video only file that have already been copied into the new file.

Is there any program that is capable of doing that or do you know a different solution to this problem?
-Atemu

(Sorry for my bad german)
I found a solution to my problem:

Ffmpeg can use a http stream as input:

ffmpeg -i YoutubeVideoOnlyURLHere -i YoutubeAudioOnlyURLHere -c copy -map 0:0 -map 1:0 Path/to/outputFile.mp4

You can get the needed URLs from
youtube-dl -f bestvideo,bestaudio --get-url https:www.youtube.com/watch?v=XXXXXXXXXXX