Using 2 different video files one for mobile and one for desktop

Hello I made two video files one for my mobile site and one for my desktop site.  I looked through the forum but still cannot figure it out.  Could someone help me?

Same here...

There its a big thread that covers a bit of it here:

And I have added my question to the end of this long one. It seems to be filled with people like us. You may want to post your message there.