Lets go back to basics. If you have it available, look up the keyboard in section 4 of the IBM AT Technical Reference (6280070). Invaluable book. You need to know what the keyboard is doing in two-key situations then handle it accordingly.

The standard AT keyboard stores up to 16 keystrokes internally, and will burst them out when it can. You might have to modify your read routines to accomodate multikey outputs.