Duo Hardcore is a mod mainly meant for Hardcore Experience.
Features/Mechanics

It has various mods such as Hardcore Rescue mod or revival should you get downed.
It has a system in place should you die without getting revived you will be timed out for a set period of time which can be configured by each server on their own. That way you are never really out. But there is a catch if you die you lose everything and everything.
It also has various mobs that are out to kill you constantly.
It has Smarter Vanilia Mobs who will do all sort of things such as a creeper blowing up your base just because it can and it doesn't have to be near you.
It has Twilight and Aether.
It has also a new world generation mod that makes the biomes and landscape better to look at.
It has also redesigned caves and dungeons and mineshafts and much more such as the Ender and Nether
It has spice of life to increase the permanent health points so you can stay alive much longer in Hardcore mode.
It has Farmers delight for better food and cooking
It has Create to Create various contraptions and automated machinery steampunk like experience to a degree.

The Mod is constantly getting updated and it's meant to get harder and harder. The mod is simply made to kill you. With you and your friends trying to survive