This is a simple suggestion that should lift up neglected 2-piece sets (at least to a degree), require minimal coding, and not break the ones already seeing use.
Legendary Gem: While you have set items equipped in both your main-hand and off-hand and other no set items equipped, every legendary item equipped increases your damage dealt by {3.75}% and reduces your damage taken by 2%.
As seen in the comparisons below, 3.75% may also not be the right number. From the comparisons, you’ll see in most cases the 6-piece sets still outclass the gem - sometimes narrowly, and sometimes dramatically. It might be worth considering 5% instead (13,000% total instead of 9,750% in each comparison below).
Comparisons of 6-piece sets vs. gem idea:
Vyr's Amazing Arcana and Chantodo's Resolve
Vyr's Amazing Arcana is still preferable for **Chantodo's Resolve** since each stack is 100% and you can get _over_ 200 stacks (+20,000% damage already), as compared to 13 item slots for 750% each (+9,750% damage). That said, it becomes a more meaningful trade-off between the better damage from the 6-piece set or less (but still more than current) with the 2-piece set and the gem.
Wrath of the Wastes and Bul-Kathos's Oath
Wrath of the Waste adds 10,000% to Whirlwind, compared to 9,750% from this gem, and comes with some other benefits (Dust Devils rune and boosts to Rend), making the two close options to pair with **Bul Kathos's Oath** (considering the gem would mean sacrificing one other gem that could have paired with Wrath of the Wastes).
Raiment of a Thousand Storms and Shenlong's Spirit
Raiment of a Thousand Storms still dramatically outclasses the damage this gem could provide to Dashing Strike (60,000% vs. 9,750%) and likewise outperforms it on spirit generators after Dashing Strike (24,000% vs. 9,750%), although the gem outperforms on spirit generators without Dashing Strike (9,750% vs. 6,000%). That said, I don't know that that corner case leads to a particular build. But the **Shenlong's Spirit** set isn't inherently tied to a particular skill, so it may be possible that the extra item slots opened up make this conducive to another skill combination. This is the one I think is least likely to create a viable alternative using the gem, but I could be surprised.
Natalya's Vengeance and Danetta's Hatred
Currently Natalya's Vengeance is the best combination for **Danetta's Hatred**, and this boosts damage by 14,100%, but it requires very strict gearing choices. While my immediate suspicion is that this gem still would not create a viable alternative, as the Natalya build can leverage Captain Crimson's Trimmings for another damage multiplier, damage reduction, and substantial resource cost and cooldown reduction (all of which are incredibly valuable if you are trying to Vault repeatedly and contend with higher GRs). That said, this is the one I think would be second least likely to see a meaningful lift with the gem versus its current situation. (Its current situation also needs significant attention to Vault and/or Danetta's Hatred themselves, as even with the Natalya approach, you would be hard-pressed to clear even GR90 with maximum rank Caldesan's in all slots and Paragon 10,000).
Helltooth Harness and Manajuma's Way
Helltooth Harness is most commonly used with **Manajuma's Way** currently. Notably, Helltooth Harness does not actually boost the damage of Hex (Angry Chicken) at all. As a result, it's hard to make a comparison, but it seems reasonable to say this could open up a new build direction by boosting Angry Chicken and letting Witch Doctors experiment with that.
Roland's Legacy and Norvald's Fervor
Similar to a few others, even the best combination has little synergy currently and, as a result, leaves significant room for improvement for the 2-piece set. **Norvald's Fervor** can be tacked onto Roland's Legacy or Thorns of the Invoker for certain farming situations. However, neither provide a meaningful boost to Steed Charge. This means the gem has no current point of comparison but could possible open up a new build.
Bones of Rathma and Jesseth Arms
The most natural combination with **Jesseth Arms** currently is the Bones of Rathma set. Bones of Rathma provides a +4,000% damage bonus but also provides significant cooldown reduction for Army of the Dead, which has a comparative damage value against Skeletal Mage of +3,000% (12,000% / 400% = 3,000%). Thus, the proposed gem would have to rely on a higher natural damage mutiplier to account for the increased cooldown, or would have to eschew Army of the Dead in favor of other skills if it cannot compete on the grounds of cooldown. This works out, as the multiplier on the gem as proposed is +9,750%.
No love for Crusader’s and Norvald’s set or Necromancer’s and Jesseth Set?
Hah, you figured me out. 
No, I just completely forgot to write comparisons for them to be honest. I’ve added them now.