The way Sleipnir is such a singular and unique case that no one can possibly be compared with Barnabas' beloved horse... But also something something the only one Barnabas can be happy with is his build-a-waifu.

Dion will listen to critique, probably from anyone but Barnabas, he's fine. If he finds something reasonable and useful in it he may privately consider that. Because he too is not above being petty about his rival but does care about doing his very best.

Well as long as they keep it even the old man yaoi seems to be doing fine.