Will two charging cables charge my laptop twice as quickly?
is it possible to plug in two power cables into the Mac?


No.

The MacBook will use the charger that provides the most amount of power. 


  In the event that the Mac is unable to allow that amount of current in
  at once, is there a mechanism to limit how much is allowed in, or else
  cut it off entirely?


The USB 3.1 power delivery specification calls for the negotiation of charge to be delivered.  If you plug more than one it, it will simply refuse the one providing the least amount of power.