Viking Berserkers. Persians even, had troops that could dual-wield quite effectively. When you put double the weapons in someone's hands they can strike faster, that's the basis. While yes a shielded opponent will MOST LIKELY win against a dual-wielding one, it would depend on training and how much experience they have.
And if you don't know, the spear/shield combination was usually for the Phalanx and formations. Outside of that, especially while fighting alone, or after the formation broke(Either due to be overrun, which is neigh impossible because Spartans give literally no ***** ) or to engage in more 'personal' combat a Spartan would want something a bit easier to control and not be parried away than a spear, so their Swords were used to great effectiveness.
I'm pretty sure Viking Berserkers didn't generally use two axes, they used one large axe. And yeah, Spartans didn't route cause they were ******* boss, but still, even when using their swords for individual combat, dual wielding blades is almost the least ideal 'loadout' for being a warrior. Spartan blades weren't particularly strong, or designed for parrying, dual wielding is basically asking for you to get stabbed in every way possible.
I didn't say every Berserker did it, but it happened. Their entire shtick is "Lets take these drugs so we don't feel **** and go ham" I'm saying there have been warriors through history who did dual wield, to varying degress of success.
Well I mean, yeah, vikings were generally just a bunch of farmers who said " **** these Christians" and pillaged their gold every winter so they could pay for **** . There wasn't really a standardized method of armament, it was basically "Bring whatchu got".