A MacBook can actually have multiple power supplies plugged in at once and will use the more powerful one. I bet having two of the same wattage would work fine. It also works with the new MacBook Pros with MagSafe.
In fact, if you plug the type-C end of MagSafe cable into the MacBook, it will "charge" itself. USB-PD is pretty great.
It's too bad the Mac mini can't be powered over USB-C though.
