First of all, I connected two servo motors together using two of the converter cables, with the brick-like plugs connected in the middle. By spinning one motor, the other also turned - so creating a simple generator. This is fairly simple physics, and was only the beginning.
I tried out the different orientations that each plug could have with one another, and discovered a useful piece of information - by connecting the RCX style plugs in opposite directions you reverse the ploarity of the motors - so one motor will spin clockwise, and one anti-clockwise. See the Table below:
Note: The direction of the cable is the opposite of where the cable conects to the RCX plug
This is still fairly simple, but can be quite interesting for beginners to the NXT.
Next (pun intended ;-)), I connected a third converter cable to the other two, so that I could plug the two motors into the NXT but use only one port. Then I wrote a simple program using the standard on-brick programmer on the NXT brick to power motor port A. This was a success, and both motors were powered.
The downside to this? The built-in rotation sensor in the NXT servo motors will no longer work while using a combination of these converter cables - so all measurements must be in seconds (e.g. power motor port A for 10 seconds). Also, I wouldn't recommend using more than 4 motors on one port, but I am not too sure as to what would happen if you did, as I don't want to risk my NXT brick just yet!
But there you have it - it is possible, and fairly easy, to have more than one motor per port on the NXT, without having to make homebrew cables. And as you get 3 of these cables in the NXT Education kit, it is also quite practical.