Yeah, if you're going to TWF it should be two shields, or if not that, heavy shield in the main hand and a kukri or some other light weapon in the offhand. W/ a 18-20 base crit range light weapon and keen, Bashing Finish at level 10 gets very nice. I still prefer 2 shields, but heavy+kukri is decent.
But definitely heavy in the main hand. You want a heavy shield, the damage boost when spikes and bashing are thrown in over light is significant, and it's +1 AC. But if it's in the offhand, that's an extra -2 to hit for your non-shield weapon. Obviously, if you're doing 2 shields, you can use 2 heavy shields no problem. Shield Master is awesome.