I didn't really have the time to write it all out back when I wrote my tweet (I wasn't expecting to get any traction lol), but I think OP is.. kindof in the right idea, but the wrong execution. I think dumbing it down to just "the second member inspired the first" is pretty boring and not the best way to look at these dynamics that have been built up by the game. It's also just, as you said, pretty bad about Leo/need in particular. The crux of the band is that they're together. There's a lot to think about in regards to the way these characters push one another's development or in regards to how they're meant to be similar.
So I agree with you that it waters down the existence of the unit. Don't get me wrong.

As for the "split" in Leo/need, it's moreso they're the "defaults" ... the unit is extremely tightknit and by far the closest in the game. Everyone is childhood best friends, lol. The 4=1 is emphasized. However, in regards to this discussion, Ichisaki and Honashiho are the "split" duos. While it is significantly less important compared to a unit like VBS who lives and breaths and dies by it, 1+2 and 3+4 is still important to Leoni and does in fact still exist.

Ichika and Saki are the "composer/lyricist" of the unit. Songwriters. They have more "prehistory" with the deal of Ichika being the one who stayed by Saki and still visited her in the hospital in MS. Ichika and Saki still had some connection. They are synced so well, they can understand each other to a deep level. Saki composes using her past feelings of pain and isolation, and Ichika wants to understand that same pain and loneliness Saki had, being the person to read Saki's journal during these times. The development one goes through extends to the other's. There is no composition without it's lyrics.

Honami and Shiho are the "rhythm" of the unit. The drummer and the bassist. In contrast to Ichika and Saki being together, Honami and Shiho were alone and isolated, they didn't have anyone else with them. Honami faked her feelings and lived on, and Shiho stowed away on the rooftop. There's the whole deal of "selfishness" "kindness" and "courage" that is displayed especially in Honami's events time in time again, with the buildup ever since main story. It's built entirely on Shiho's words to Honami.
These characters are built as narrative foils. They've lived deeply personal experiences that impact the way they function and think, and the other usually drives this development. On a more minor note, generally events that are focused on a singular pair (see NSNF/Parallel Harmonies or Unwavering) tend to lean into the 1+2/3+4.

I'd also like to add please do not read this as "you're downplaying the other dynamics!" because no, they have plenty of great moments. The purpose of this is to describe the 1+2 and the 3+4.
Shiho's dream drives the rest of Leoni, but especially Ichika in their first arc. Ichika's impulsive side is shown pretty strongly when it comes to Shiho, taking charge to tell Shiho they want to be in a band together in RWY. For something else, take Ichika's Sakura mixed event, where Ichika reflects on Shiho's words to put on a performance that can move someone's heart. This is something that drives Ichika.

On a personal note, Polyneed is probably my second favorite ship overall. I think about them all and their individual dynamics on a day to day basis. I hope this is satisfactory.