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

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25),  Control_R (0x6d)
mod1        Alt_L (0x40),  Meta_L (0x9c)
mod2        Num_Lock (0x4d)
mod3      
mod4        Super_L (0x7f),  Hyper_L (0x80)
mod5        Mode_switch (0x5d),  ISO_Level3_Shift (0x71),  ISO_Level3_Shift (0x7c)