[spookthesunset]

One problem with two separate designs is deciding when to show one vs. the other. This gets especially tricky when people share links. Wikipedia, for example, has two different URL's: one for mobile and one for desktop. How often do you get links to the mobile version instead of the other?

And if you keep the URL the same but serve different output depending on the browser, then you get inconsistent behaviour between two different devices.

Nailing the UX for mobile and desktop is actually pretty damn hard.