I'm not totally sure how this is done on the PSR-740, but it can be done. I had a PSR-520 a long time ago, which was sort of part of the same general era of Yamaha arrangers, and as I remember, there was a main menu, and you could step back and forth through the different functions of this menu -- and one of the options was the voice and voice characteristics of the split voice, which is what Yamaha calls the voice for the left hand side of the keyboard.
Another thing to check is to see if your keyboard has a button labeled "Split" -- if so, then usually, pushing it turns the split keyboard feature on and off, and if you press and hold that button down for a couple seconds, it may take you right to the options to select the voice you want for the split/left hand voice, as well as its characteristics, such as volume, octave, reverb, and chorus.