They're canonically in love in the novel and it's not
selfcest cuz they're not the same person at all, they will separate and the reason why they're in one body will be explained later, grandfell even called hoyeol "my dear" "my light" and sometimes hoyeol also called grandfell "my grandfell" oh and they also thought of hiel, diend, gwicheol as their children in the novel but those details are deleted in the manhwa so i suggest you read the novel too cuz the manhwa cut at least 90% of the bromance between them and there's also many changed/deleted scenes, mischaracterization and fan service so it's better to check out the novel too for the lore and bromance.
Currently at ch 121 and I've read a few comments about bromance between Granfel and Hoyeol, as an avid enjoyer of action bromance I'm trying to wrap my head around the idea cuz I thought they're somewhat the same person so I'm kind of like is this selfcest or Smth else I don't mind spoilers so if anyone can explain their relationship if would be greatly appreciated