I'm guessing you haven't ever seen Xayah and Rakan in play, since you're only posing hypothetical statements.
Riot took a careful approach at designing Xayah and Rakan by giving them just the smallest amount of synergy IN their kit, while letting their playstyles do the rest of the synergizing. If they're together, they can sync their recalls, Xayah can give Rakan her Deadly Plumage buff, and Rakan can Battle Dance to Xayah from a longer range than with everyone else.
This allows them to tune both of them individually while also having a secondary tuning lever to look at, AND it helps them learn what cool new stuff they can do with champions. Who knows? Somewhere further down the line we might have lorefoes with designed strengths and weaknesses against each other.
But, enough rambling about the good side of it, let's look at how this translates to your hypothetical statements:
A. They are subpar and under powered when played without the partner, and needs buffs that don't impact their potential as a duo.
In statement A, we assume that Xayah and Rakan are underpowered alone, but impossible to buff because of their kit synergies.
Rakan was released A.O.K., though he did receive some small buffs/rebalances, and they're still working to fine-tune him in the direction they want him to go. And since the kit synergies go only to Recall - Lover's Leap and Battle Dance, they can holistically look at his kit to see where they need to buff him.
As an example: It's safe to buff the shield on Battle Dance (since the only effect Xayah has on it is its range), while also nerfing the heal on his Gleaming Quill and increasing its range.
The same scenario goes for Xayah, (although IMO she was overtuned just slightly on release, but that's fine because she wasn't overpowered). Since her Deadly Plumage is only given to Rakan when he's around, her kit is just as easily looked at and tinkered with to produce good solitary nerfs/buffs while not impacting duo performance.
OR
B. They are overpowered when combined and need nerfs.
This hypothetically could be a problem, but only if their entire kits was designed to synergize with each other. And, as mentioned before, the synergy in their kits is limited to Battle Dance, Deadly Plumage, and Recall - Lover's Leap. Because of this, their duo synergy can't overshadow their solitary play - or in the off chance that it does, they can easily make targeted nerfs to the pain-points in their kits without impacting solitary performance.
Unoriginal Riot couldn't come up a with 2/1 concept like Blizzard, so they tried to copy the duo idea, the only difference being that Cho cannot be played without Gall, making it easy to balance. They are balanced as one item. You can either nerf Cho, or Gall.. but regardless of which you nerf, it will affect both of them the same.
Maybe, juuuuuust maybe, Riot wants all of their Champions to be playable alone? They know that some players won't be able to have a duo partner available to play whatever two-in-one Champion they make, and don't want to touch on that space because it'd exclude a lot of people from the fun? I don't know exactly what their reasons are, so I'm mostly just spitballing this, but I'm pretty sure Riot doesn't want to do a two-players-one-Champion thing because, for the most part, it'd leave a lot of players out of the fun of playing that Champion.
Besides, it's not like all of their designs have to be compared to Heroes of the Storm - especially since HotS takes up a pretty radical design space with their Heroes, like The Lost Vikings being a three-in-one Hero that requires a lot of management skill, or Cho'Gall being a two players, one controller-esque deal where coordination can make or break it.
Altogether, Xayah and Rakan are a "pathetic attempt at Cho'Gall" because they're not an attempt at Cho'Gall in the first place.
