Interestingly, I found the following thread on the other forums by accident, and it seems to suggest that many more than 2 keypresses can be registered (apparently QWERTYUI works):
Yes, the 2 is not a maximum, it is a minimum: it can register at least 2 presses, often more. If you look at the diagram Neelix has created above, all keys in a single row or column can be pressed provided they are the only keys being pressed. Also, you can press any keys that do not share a row or column line, so Enter, 2, E, D, C, and Fn could all be pressed and registered, but the very next key would necessarily fall on an existing row or column and could screw things up.
You can also mix and match these combinations: You could press Q, W, E, R, and T, 6 and 7, and J, N, and M. The maximum number of keys you can press and still have register would be Enter, L, K, P, O, Del, 0, 8, I, J, N, and M. Physically, anyway. The software may have some issues registering that many keys.
The 2 button minimum comes from when you push buttons on the same row/column. If you press 1 and 2, everything else in columns 7 and 6 will break the matrix; you can still press anything in columns 0-5 though.
The matrix of most desktop keyboards is laid out such that you have a minimum of 3 key presses, but they have far more buttons and require a more complex matrix.