If I had a way to strike-through text, I'd just say "give nana a grab," strike it through, and be done.
Popo needs an actual recovery if Nana is KO'd.
One way to possibly make them more viable (yes this sounds ridiculous) - make Popo a little heavier, but give Nana a weight of like 140 or something really high (I think Bowser is currently 180?). Then targeting Nana is tricky because she's not gonna be KO'd nearly as early as Popo, and so you get the benefits of two climbers longer.
I also suggest that mid-air taunting instantly teleports your Nana to you (no taunt animation) if Popo is not in hitstun or tumble and Nana is not in hitstun (could be in tumble). This will let you save Nana quicker and also means that even if your opponent knocks you off (and leaves Nana on stage because she's so much heavier), you can get her back way easier.
Both of these ideas might be useless, but they could work...