There are a lot of players that are fairly "equal" between Bayern and United. But I'm very surprised that you said you wouldn't swap Upamecano (who is a walking catastrophe) and De Ligt for Martinez and Varane. Very few general football fans would share that view. Rashford is clearly better than any Bayern wide forward/CF and Casemiro is literally a contender for greatest DM of all time - not including him must be trolling.
Picking a combined 11 would be pointless because Kimmich is obviously the best RB between the two teams so in theory he could go there but we all know that he would refuse to play there.
The outfield (not including keepers cos we don't have one!) United players that would "walk" into the Bayern side: Martinez, Varane, Casemiro, Rashford
The outfield Bayern players who would "walk" into the United side: Kimmich, Musiala, Pavard (who you are trying to sell)
Then there are fine players that I couldn't choose between (such as Shaw v Davies) or players that would get in depending on formation: Goretzka, Muller, Bruno or fitness: Martial, Coman.
Both teams have good defences, decent midfields, and poor forwards so it isn't easy to combine them. And both teams would drastically improve with the addition of Kane.