What would happen if I plug in two USB-C power supplies into the same Mac? Would it charge faster?
No.

Your MacBook will charge from the one which provides higher power. Quoting from the Apple support document, Connect with Thunderbolt 3 on your new MacBook Pro:


  Notes on charging your MacBook Pro:
  
  
  Your MacBook Pro draws power from only one power supply, even if more than one is attached—so using multiple power supplies won't speed up charging.
  If you connect multiple power supplies to your MacBook Pro, the one that provides the most power will be used, regardless of the order in which you connected them.
  You shouldn't connect any power supply that exceeds 100W, as it might damage your MacBook Pro. Also, using a power supply that doesn't provide sufficient power can result in slow or delayed charging. It's best to use the power supply that came with your Mac.