Units with two melee weapons always use their secondary weapon to charge. The Ai is not able to handle these correctly. The only units that make correct use of primary and secondary weapon are the phalangites.

A related issue seem to be units with prec missles always start an attack with this weapon, even when they are already in contact with the enemy (any enemy). So you see the AI spearmen on "reloading" while the front rows are already been cut down by your swordsmen. The same happens when the spearmen are already fighting with one of your units and you send a second one into the melee: the AI changes back to "reloading". Even more: when facing two or more enemy units at the same time the "reloading" seems to be endless, lasting until the unit is routed.