You can already dual-wield: if you have a weapon in each hand, you can block or attack with each for an action. The idea of hitting somebody with both weapons at once is something for staged choreography and fantasy roleplay: you don't gain anything by doing so, and you can't hit from two different sides at once with any force, and it's really clumsy and self-defeating. This is the same reason it's not physically possible to throw two javelins/rocks in one action with any effectiveness--your body simply doesn't move that way. You generate power for your strikes in large part by rotating your shoulders and hips, and you can either rotate your left side forward or your right side forward. You can thrust yourself forward square to your opponent and generate power that way, but (a) this is a bad idea, since you have shorter range and make yourself a larger target, and (b) you're still only going to hit from one direction.
There is quite a lot of historical material on using two weapons at once, not from the target period but from the late middle ages onwards. You'll see in these materials that people either strike with just one of the weapons, using the other to defend, or strike with these weapons in turn. The system already simulates the historical use of dual wielding, so nothing needs to be changed.