Convention of Elements vs Skull Grasp?

Coe cubed with f&r

skull grasp sucks because the Dust Devils passive from the Wastes set is the main source of your DPS for WW barbs, and skull grasp doesn't increase that damage

If you are going for higher grifts, use a combination of CoE/Unity/Obsidian Ring of the Zodiac between your 2 slots and cube

If you are going for speed farming, use Focus/Restraint and cube CoE

How do you increase that damage then?

you follow the advice above

this, depending on what u need

Would you still use unity if you're rifting with 2 people?

Definitely, but only if the other person is also wearing unity. It's best to have both of you die every 10 seconds to make it more challenging for you.