xlr et booth output - individual volume ???

hello,

i appreciate if some one can help me resolve this problem.
i have my rmx2 usb conected to a laptop. from the rmx2 i`m getting out (main) through xlr output for my enthire system (inside) controled by main volume knob. now i wan to install another exterior system that i want to be able to controle it`s volume separatly from the console. so i have to send the signal out from the rmx2 through the 1&2 booth output. how can i control those 2 system independent one from each other (volume) ?
appreciate any advise.