xmodmap:  up to 3 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x4e),  Caps_Lock (0xda)
control     Control_L (0x42),  Control_R (0x69),  Control_R (0x6d)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Alt_L (0xcc)
mod2        Num_Lock (0x4d)
mod3      
mod4      
mod5        Super_L (0x25),  Super_L (0xce)