[RPG] What are the advantages and disadvantages of dual wielding in 13th Age

A Dual Shield fighter is possible, but is restrictive and fiddly.

First, when making high-concept stuff like this, it's critical to be aware of potential pitfalls of your character. Take a look at this guide to get a sense of the traps involved.

To realize this, my first preference would be to refluff some more "standard" version of the fighter into a double-shield fighter. Holding two shields does nothing for your defensive capabilities. But, with that said, the feats of superior proficiency "spiked shield" and midnight blade student make dual strike quite... interesting. You can "shield bash" two people a turn. Of course, the problem is in good positioning and actually doing damage. While entertaining, the dual strike fighter tends to be severely "one-trick pony"-ized.

As a first time player, I would instead look towards the Knight-variant of fighter. There's less "fiddly" stuff, and the idea of a heavy shield + spiked shield can serve you quite well. It's also very very hard to go wrong with Dwarven Weapon Training, with a Heavy shield and "gauntlet axe" (if you want to fit your personal style) and just say the "gauntlet axe" is a razor-sharp shield.

Two shields as "important" only really comes into play with the Snapping Tetsudo Paragon Path, a relatively uninspiring choice, unfortunately. To be fair, Dragon Magazine 385 has a significant number of "shield-as-weapon" choices, but few of them require two shields. All that being said, I would still go with knight + dwarven weapon training, and either pretend a hammer or an axe was a shield, depending if you want to be bashy or slicy. The mechanics fit better, and the thematic imagery can be just as vivid.

The question will be a real issue in case of multi-classing, since Combat Boon is a Cleric's power, while only Fighters and Bards have flexible attacks.

My conclusion is:

Flexible attacks are not basic melee attack

Hence, you can't use Combat Boon with Flexible attacks, but I would rule Hammer of Faith as "yes" because it is a passive buff, and because it's a Daily. The game goes out of the way to make Daily powerful (transparent targeting being the first thing I can think of).

My reasoning is base on the text for hampered, page 172 of the rule-book.

You can only make basic attacks, no frills...(Fighters and bards, that also means no flexible attacks) [Emphasis mine].

Also, additional attacks per turn granted by powers or talents (the Fighter's Counter Attack, for instance) cannot be used as triggers for flexible attacks. This reinforces that the two (basic attack, and flexible attack) are two different beasts.

Hence, I guess the intent of the rules is for flexible attacks to be considered as powers with a chance of triggering depending on the dice roll. Essentially, I guess my stance is "only one effect from one attack" - which I would argue it's the rules' intent because you can only trigger one flexible attack per trigger, despite being eligible for more than one.