First it is certain that you would not want to plug both outputs of the stereo power amp into one speaker combining both channels into the same speaker. (see image from manual below - also link to manual)
Next I would be very concerned about only using the one side of the main amp out as it could cause an unbalanced load, or you could be driving the load into dead air connection and any of these conditions are destined to cause premature failure of the power amp.
Taking a quick scan of the manual for this unit, it appears what is pretty common to most powered mixers, hook up two matched speakers/loads to each chanel of the output.
Your best bet is to shed those monster cabs, there really is no need for them especially with this sized powered mixer, and get yourself a nice pair of full range cabs like EV1202;s, EV SX300's, or some nice 2 or 3 way 15" JBL's/
Just my opinon.
Link to Yamaha EMX 312 Owner's Manual:
Yamaha EMX Series Owners Manual