Is Data Transfer between 2 Ubuntu Machines possible via USB-Cable?

When you say, USB cable you are referring to a Windows Easy Transfer Cable, right?

In that case, the easiest way to do this is to just hook the two machines directly together with an Ethernet cable. Modern NICs are smart enough that you don't even need a crossover cable; they just automatically crossover when they detect that you connected two computers.

If you absolutely had to do this for a proof of concept you could simulate the ancient art of "laplink" transfers by emulating a serial port on each machine.

Usb to serial on one, and usb to serial (null modem) in the other, sorting out gender issues in the middle.

It would not be quick.