Arslan/Daryoon is heavily implied. It's like Alexander the Great & Hephaestion with roles reversed.
While I love you for mentioning Arslan, the trope is not implied as much as it's entirely unintentional. The author is famous for not liking Ho Yay - strange when one considers that he came up with Reinhard and Kircheis as well.
The anime's character designer, Kamimura Sachiko, has done a series of BL illustrations featuring basically expies of Darun and Arslan. She denied any connection but it was pretty much a case of "well, just because one of them looks exactly like Darun and wears exactly the same outfit, and the other looks exactly like my version of adult Arslan, right down to the clothes, jewelry and weapon, doesn't mean it's them!"