Just because it's not clear who the main focus is doesn't necessarily mean multiple artists should be credited. The only evidence I have seen of multiple artists working on the same key art is Kagome's comment on the key art shown in post #12078337, implying he collaborated with another artist, presumably Motoyon. However, Suzuri's comment on the key art shown in post #12080127 implies there was no collaboration on the key art of that piece, even though the focus is about equal between Kimika (Suzuri) and Takuji (Motoyon).
Still, the general rule is that if the focus is even between two or more characters whose key artists are different, unless something suggests otherwise (like Suzuri's aforementioned comment), they should both be credited.
These guidelines were determined by reading the Subarashiki Hibi Official Visual Archive and Subarashiki Hibi ~Furenzoku Sonzai~ Official Artworks (2018), and is by no means conclusive on which artist drew which CG.
