Splitting power from usb type c charging brick
There is no such power splitter on the market that I have seen, and if it did, would violate the USB spec and defeat the purpose of Type C PD spec.

But technically, you could do this on your own but may not be worth it. You could just tap the VBUS and GND lines of the power adapter and tap power from that.

You will not get any voltage unless you plug the adapter into the laptop. And because the laptop is the device in control, it could ask for 5V, 9V, or 12v (or even 20V if a 20V charger was plugged in), so you might not get the voltage you were expecting. Also, in the standard, the device knows the capabilities of the power adapter, but your additional load throws that off and the total load can overload your adapter.

USB Type-C has complicated power negotiation. If the laptop or other device is not plugged in, it will turn off its output for safety. The default voltage is 5 Volts. Higher voltages require special signalling between the device and power brick. And the laptop could tell the brick it no longer needed power and voltage would fall to 0.

Anyway, that should give you some idea on the ramifications of what you are asking for. Here's some technical info on it:
http://www.ti.com/lit/wp/slyy109/slyy109.pdf