Will a single stack 1911 upper fit/function on a double stack 1911 upper?
Upper what? A 1911 has a frame and a slide. Yes the slides are the same for the standard and high capacity frames.
Brilliant "Skippy" Answer
No idea what you're asking Flapjack, An upper on an upper? Wtf, or like side by side, maybe you could try this? https://www.youtube.com/watch?v=XCOQf12y3kw
Only if the barrel is 5 inches.