Does the new 12" MacBook (2015) allow for dual displays, or just display mirroring?
You get two distinct, functioning displays unless you choose to mirror. Same with AirPlay.

From http://www.apple.com/macbook/specs/ :


  Dual display and video mirroring: Simultaneously supports full native resolution [2304 by 1440] on the built-in display and up to 3840 by 2160 pixels on an external display, both at millions of colors