I don't know the exact terminology and I only glanced at your links but if your doing what I think your doing this might give you an idea to the problem.

A keyboard uses around 5v (I believe it varies from 4.5 - 5.5 volts) to :

A) Power the keyboard (LED's etc).
B) Send the key press signals from the keyboard to the pc.

In very simple terms, if you connect 2 PC's to one keyboard directly you will be sending twice the voltage to the keyboard and also risk 1 pc sending 5 volts directly into the other pc.

They may only be small figures but something receiving an extra 5 volts can cause damage

EDIT:

My current workings are based on general theory and experimentation and eventually I hope to make my own vetra box. The methodology I believe involves sucking out the power sent from the pc to the keyboards via some basic circuit and then introducing an additional power source into another circuit which then controls the signal/power being sent to/from the keyboard and back to the pcs.

Of course I really don't have time to explain my current works in full until I get it working but one day I will add a home made multiplexer solution (I'm sure).