xmodmap:  up to 2 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x40),  Shift_R (0x44)
lock        Caps_Lock (0x41)
control     Control_L (0x43),  Control_R (0x46)
mod1        Mode_switch (0x42),  Alt_R (0x45)
mod2        Meta_L (0x3f)
mod3      
mod4      
mod5