how can i put both outputs on master? (omni control)

hey guys. i have two studio monitors and one subwoofer but i can't connect all three to the master. any of you know how to set both outputs to the master? if its possible of course. or if not , what do u believe i should do? i was believeing of buying a cable splitter in order to put both monitors on L side and the sub on the R side. need some advice really.

thanks! ''
Paul