It dawned on me recently why that probably is, and it's rather basic and obvious storytelling. One custom figure is just that... it's a design, and for all the thought I put into it, no matter how clever a name I come up with for it, it lives in a little vacuum in my mind.
But as soon as I start on that second figure a relationship exists, and because there's a relationship they begin to flesh out as unique characters in my mind. How are they alike, how are they different? How do they regard each other? So many gaps are filled in almost instantly.
As soon as I decided the Mirialan (the gal with the green face) figure's lack of facial tattoos meant she was an untested rookie, I knew the Arconan soldier was a veteran of a few conflicts. She's young and eager to prove herself. He's older and more concerned with simply surviving the next engagement.
This thought process helps me decide on details to add and color schemes to try. Trying to tie all these concepts and visual cues together inevitably inspires a third character, or fourth, and all the while the relationships are connecting and inspiring more ideas to work into new characters... until I grow tired of the initial concept or decide I've "told their story" fully enough in my head.