AuraTwilight
The Sorcerer of Smiles
- Location
- The Silver City
The reason that Madoka has that huge potential is not because Homura cares about her (per se), but rather that her death is the linking point of all the different timelines so she achieves a huge importance. But her death is directly caused by Walpurgisnacht in most of those timelines, so Walpurgisnacht is also hugely important (though not quite as important as Madoka).
Just gonna ignore the usage of 'single point', then? Okay.
While I haven't read that manga, Madoka's potential means that she is in a league by herself when it comes to strength. Presumably, soul gem capacity is a part of that strength. So Madoka "splitting the magical costs" with someone who lacks her soul gem capacity sounds like a horrible idea and not a good indicator of how expensive the attack would have been for Madoka if she had done it alone.
It's enough to witch her out instantly, as Homura says herself from experience, so she tries to take the load, telling Madoka she'll do it 50/50 when she tries to take most of it.