So after 2 days passing over this post i finally understood your meaning, and seems i'm not the only one in that case.

Answer is no you have to switch your left weapon in inventory to replace shield by old selection system, no short cut or any macro.
And no you can't have a left weapon that goes only in left hand, simply because left hand usable weapon are also usable main hand, and it's certainly hard-coded.

Pirith.

the /use2 macro is working for that on uthgard, i did it like that with my imba-staff to change weapon and it worked, so it looks like a little bug =)

nope, doesn't work, /use2 still wields it in right hand slot