What did you already try out to solve the problem? We always try to help people here in the forums, but most probably nobody wants to do the whole engineering work and coding for you.
As you have for sure already analyzed, you will have to write software for the 1st PC which grabs the mouse actions and sends them to the Teensy over USB/Serial. The second step is writing code for the Teensy which decodes the incoming mouse messages, translates these and sends them again out to the second PC.