xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock
control     Caps_Lock (0x42),  Control_R (0x69)
mod1        Meta_L (0x40),  Meta_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3        Alt_R (0x25)
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  BadKey (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Hyper_L (0x87),  Mode_switch (0xcb)