Short-circuit power outlet with two macbook chargers and one cable
Nothing will happen.

You are not directly connecting anything from the mains side of the power adapter to the USB side of things.  The low voltage side (USB) actually isolated from the high voltage side.  The highest amount of voltage being passed is +/- 5V which is more of a data sense connection.  It’s looking to see if there is a device making a request to be charged.  Since power adapters don’t make this request by design, it will never send higher voltage. 

Now, it's not fair to say absolutely nothing will happen as it's conceivable that you might blow the USB controller (the chip that does the negotiation). However I would bet that the engineers designed the circuitry to be one way only. I don’t have access to the schematics, but I would bet there are diodes on the 5V connection’s to ensure a one way connection - they are there on the older MagSafe adapters so it only makes sense they would still be there on the newer  USB adapters (see above link)

From a pragmatic point of view, engineers would design to accommodate this sort of behavior - they design things to fail safe to the user not fail to protect itself.  That said, don’t do it because the shock you'll get is the one that comes from having to replace two previously perfectly good power adapters.