In my militaries, I almost never use more than one squad just due to having low populations, and in each squad I try to have every dwarf have a different weapon. After going through the five weapons dwarves are actually intended to use, you start to run out of weapons, and I've actually used two shield dwarves before. I can't remember if I've ever paid attention to one in combat though, but I think I would probably remember if they all died on their first fight though, so they might actually be decent.
Dual wielding shields gives them more chances to block. I'd imagine at a high shield user skill they'd be almost impenetrable tanks. Shields wouldn't be as effective of a blunt object to beat people to death with as a mace or war hammer, so they wouldn't be the best at killing, but they could probably face tank almost anything one on one. A common trick in adventure mode is to stack 10 or so light shields in one hand so that you can block almost anything and everything. In fort mode dorfs can't do this, but wielding a shield in each hand is close enough.
I actually considered having a squad of dorfs dual wielding shields as an anti-undead squad. Whenever I've fought undead, armor either protects my dwarfs from minor injuries or does nothing to stop them from getting pulped by the undead super strength. Blocking and dodging seems the only defense for fighting undead up close.