when you reshape the camera viewport, its like streatching or squashing an image in a program like photoshop of the such - to retain the proper sizing you have to change the dimentions by equal amounts.
The following values will always retain the correct screen dimentions:
160x120
320x240
640x480
400x300
800x600
512x384
1024x768
More are available - and these can be found by dividing a known 'true' dimention (one that displays correctly) by 1/2 or 1/3 or whatever on both sides eg:
1/2 of 800 = 400
1/2 of 600 = 300
Do this to get the correct size for each viewport
So the simple solution is not to use the whole screen in 2 player mode, try using the same size windows as you would in 4 player - but only create and use 2 of them.
Hope that helps!
Ask if u want any questions answering.
Mog.