On page 131 of the manual it looks like there's a "Key Shift" function that lets you raise or lower the pitch of each part from -12 to +12 semitones, so you should be able to shift the Left part up or down an octave that way.
Pages 139 through 144 talk about how to edit the sounds, including changing the Octave Range and the Pitch. I gather that you can modify the sounds for each part this way, although I'm not sure.
I don't think you can quickly shift the Left part up or down in octaves by just pressing a pair of buttons, the way you presumably can with the Right 1 and Right 2 parts. However, you might be able to set up different versions of the sound you want to use for the Left part-- "normal," shifted down an octave, shifted up an octave, etc.-- and then save each version for later recall.
Do you have the manual? I found it at the Panasonic web site, although it's an image rather than searchable text. Also, there seems to be a user group dedicated to the Technics keyboards, and they should be able to help you.
Thank you. I had read the manual a number of times (you know how boring they can be) and missed this. I just a one octave shift on the bass side and this works perfectly. Got the keyboard about 6 weeks ago and trying to learn my way around.