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

shift       Shift_L (0x3a)
lock      
control     Control_L (0x2c),  Control_L (0x68)
mod1        Alt_L (0x4b),  Alt_L (0x6a)
mod2      
mod3      
mod4      
mod5      

