Lets see, lance is 600% smash damage for humans without thames.
Bash is 920% damage without celtics. On an elemental immune enemy it's clear that duel weapon bash will win. On an non elemental immune enemy the max bonus (with a single lined elemental reforge) is 55% for lances and 44% for duel weapons.
AKA 930% lance smashes and 1324.8% bashes. Lets take r6 into consideration now.
Bash crit with two ele 3 r6 weapons - 3423% damage Smash crit with ele 6 r6 lance - 2901% damage
Lets not forget than bash stuns monsters and has a quicker animation than smash. While you will have to build it up to the maximum step to achieve these results a 1st stage bash crit under the same circumstances is 2928% damage.
Basically under ideal situation a humans duel weapon bash will be better than a lances smash. Now piercing will probably tip the tables in smashes direction for some enemies but the speed of bash might actually increase it's DPS. Personally, with the current content even without piercing a bash will one shot most mobs and since the combo meter doesn't disappear when changing enemies bash just outclasses smash in every way. It might be a bit different for giants though. They wont have access to Royal Celtics and will have to duel wield blunts.