How seamlessly does the Magic Trackpad 2 pair with a Mac when plugged in? I've got an external monitor and keyboard, and I switch between two different MacBook Pros by plugging a USB and monitor cable into whichever one I need to use. I'm considering adding a Magic Trackpad 2 to the setup. 

If I keep it plugged in, and switch it from one computer to another, will it instantly and automatically pair with whichever machine it's plugged into? Or will I have to open System Preferences every time or do something else to get the new computer to recognize the trackpad?

Again, what I'm imagining is keeping the Magic Trackpad plugged in to a USB hub at all times, and switching the hub from computer to computer.
You can consider use the Magic Trackpad 2 in wired mode. Do not pair it with your MacBook over Bluetooth.

You can switch the USB hub between MacBooks and the trackpad would get connected instantly in wired mode over USB.