Bucklers were actually invented after the sort of full plate mail setup generally made large shields pointless, and people just started carrying around two-handed weapons, instead. The term swashbuckler comes from the term buckler, so you can probably infer how they were historically used from that - mostly used in essentially unarmored dueling alongside a sword, which is often the sort of situation you're getting much of these off-hand main gauche type fencing styles that we're talking about in dual weilding.
As for using it with crossbows, well, again, bucklers don't help much against arrows, and frankly, that's what crossbowmen were worried about most. Real crossbowmen often carried around things called "Pavise", which was sort of like an even larger tower shield with a kickstand, so that you could set it up as a mobile fortification in front of you. Crossbowmen lugged those things around, built a free-standing shield wall in front of them when they got into position, and then started the slow process of winching their crossbows from behind cover to protect them from the crossbowmen on the other side. Very effective in formation warfare, where you could have a reasonable expectation of not getting flanked or overrun, at least until the stuff was in the fan, anyway.
Since we seem to be bringing up the Battle of Agincourt quite a bit recently, I remember one bit from a History Channel documentary on it about the French relying upon mercenary crossbowmen to be their answer to the English longbowmen. In their haste to get to the battlefield, however, they ordered the mercenaries to leave their pavise behind, so when the mercenaries were ordered to the front to start engaging in missile skirmishing, where the longbowmen had cover, and the crossbowmen did not, the mercenaries basically said "you don't pay me enough to die a senseless death", and just turned around and left the battlefield.
As for "hey, it's totally berserkery/vikingy for a dwarf to weild two axes", well, yes, I can actually agree with that, somewhat. Having a hand-axe in both hands, however, does not mean the same thing as dual-weilding axes, and striking at the same time, however, like I said before, it's largely just a choice between whether you hit someone with the right hand or the left hand this time. A dual-axe method of combat would have some use if you were worried about having an axe stuck in the head of someone you just felled while you were surrounded by enemies - just drop the stuck axe for a while, and come back for it later.
Still, we're talking about using that off-hand weapon as a parrying weapon at best, and just something occupying the other hand that doesn't matter at worst.
Real vikings, who were well trained in ship-to-ship combat would often use some sort of axe or chopping sword in one hand, and a sheild in the other, or a hook in the other to drag the enemy vessel (or just the enemy) nearer.
If we had a system for using weapons to grapple, this might actually be a little more sensical, as you could use something like the ninja weapon, the
kusarigama, which is a two-handed weapon consisting of a chain with a weight on one end, and a sickle on the other. One hand throws the chain to tangle (grapple) the enemu, while the right hand uses the sickle end to try to finish them. Likewise, a roman-gladiator-style weighted net and sword style would make sense.
oh, and...
However if it was done with daggers it could be useful.
The problem with that is that daggers are friggin' nearly useless killing weapons in serious, armored combat. You're essentially better off grappling.