the command is "addDynamic"
try create 1 emitter,omni and name emitter1,it emitte particle1
then use 'create particles tool' ,place some particles in the scene name particle2

then type "addDynamic particle2 emitter1;" in script window

and hit play,every point of particle2 emitte particle1.

if create 2 emitter,and use "addDynamic particle1 emitter2"
can make some intereting effect..

maybe everyone knew this already...but I just learn this a week ago...learing mel command one by one....