#Tower Shields are always bad for a character
Tower Shields are always bad for a character
It is literally impossible to use a tower shield where you would not be better off using a heavy shield. See my answer here (and note that the accepted answer posits a single niche use-case that only “works” because of a misunderstanding of the rules). Only four classes gain proficiency in tower shields by default, and three are prestige classes that you aren’t using. The fourth, the fighter, has myriad options for replacing Tower Shield Proficiency with something else – which, by definition, is better.
Anyway, reducing ACP is fairly trivial; just have the shields made from mithral. The tower shield in Player’s Handbook is made of wood, but Races of Stone has a steel tower shield (which can then be swapped to a mithral tower shield), and Races of the Wild just has a mithral tower shield (which, oddly enough, appears to be the core tower shield, i.e. the wooden one, with the standard mithral modifiers applied to it).
Though I have no idea where you got the idea that having an Armor Check Penalty of greater than −1 prevents you from dual-wielding. That is not a rule. Armor Check Penalty doesn’t really apply to anything aside from the relevant skills, so long as you are proficient.
Being able to bash with a tower shield, though, that isn’t an option I’m aware of at all, feat or otherwise. Your reference to a third-party book (Plot and Poison) may indicate broader allowed material than I am familiar with, but I am about 90% sure that Wizards of the Coast never offered the option. Even if they did, again, tower shields are bad for you.
#Dual-shields, on the other hand, is quite effective
Dual-shields, on the other hand, is quite effective
I have run a couple of dual-shield characters, and both were quite effective. They both used a heavy shield as main-hand and light shield as offhand weapon, which is almost-certainly the most effective way to go about it. Dual-shield is a quite feat-intensive build, and wasting additional feats on allowing you to competently use two one-handed weapons is not a good idea.