One of the posible wild surges from the playtest barbarian Wild Soul subclass:
Shadows weave around a weapon of your choice you are holding. Until your rage ends, your weapon deals psychic damage instead of its bludgeoning, slashing, or piercing damage, and it gains the light and thrown properties with a normal range of 20 feet and a long range of 60 feet. If you drop the weapon or throw it, the weapon dissipates and reappears in your hand at the end of your turn.
By default a greatsword (other big sticks are available) has both two-handed:
This weapon requires two hands when you Attack with it.
… and heavy:
Small creatures have disadvantage on Attack rolls with heavy Weapons. A heavy weapon’s size and bulk make it too large for a Small creature to use effectively.
But if the barbarian wild surge comes up with option 7, then it seems it would gain light:
A light weapon is small and easy to handle, making it ideal for use when fighting with two Weapons.
So… going by the mechanics alone, I see no problem between a weapon being both light and heavy, since a small creature would have disadvantage and you could duel wield, but thematically I have a problem with something being light AND heavy since, ya know, there both oposite6 ends of the “how heavy” scale. I guess the important question here is how heavy is it?
The mechanical problems come with light and two-handed. Two-handed sort of rules out duel wielding unless you have more than two arms (but I’ve seen this be overruled in official books, such as when a giant wields it in one hand) but light says that it’s perfect for duel wielding. So what happens in this case?
My interpretation is that you can’t duel weild because of the weight and heftyness, but light cancels that out, but I may well be wrong.