I'd considered this a long time ago, but I really don't consider it an ideal solution. If anything, weapon combat in 5e is lacking in variety (though I'm fine with the amount it has), and this would collapse that mechanical range even further. Also, thematically, the second weapon adding damage to the first one isn't really that much more realistic, since having a shortsword in one hand doesn't give the other shortsword more killing power behind it. I guess it could model a more aggressive series of blows in totality, but the image doesn't come across very well. Plus, it pretty much makes greatswords pointless, and doesn't do anything to explain how it works with thrown weapons.