Why would you use more than 1? Those things are HUGE!!!!!!!!!
If you have a giant portable then I guess you could get away with it, if you want some REAL force feedback.
well, a good example is the GC controller, it is small and it have the rumble located at the middle.
it is a good example of a good rumble system, but when converting a normal gamecube into a laptop, the most common option is split the controller, and integrate it with the entire system, even the smaller portable gamecube I saw, them have to separated sides for controls, one of them generally have dpad and joystick, and the other one have the c joy and other buttons.
place a rumble under the screen or the middle part probably will not be as efective as place them under the "action parts" (the one you use the buttons and hold the system.
this is even more necesary on bigger portables or portables with more space bettwen the "action parts"
well, I know there are lot of people that removed the rumble feature out from their portables, but as a gamming lover I decided to keep that feature on my portable.
and yes, my portable is GIANT