Welcome to the forum....
Windows Defender is an Antivirus program. I am not sure what a video down loader has to do with an antivirus program but yes, you can have two on your computer. Usually if you install another Antivirus program like Norton, Avast, McAfee or others, Windows defender is deactivated, however, it will always be updated automatically by Windows 10.
Of course you can have as many video down loader on your computer as you like. If it makes sense to have as many, is not for me to decide, it is your computer after all...