I'm deeply engrossed in this story right now; it's such an unusual but well-done crossover that I simply loved how it develops the personalities of the two characters, so I'm here sharing it.The best I can do for anyone interested.
Kenji Watanabe designed Beelzebumon as a Dark Hero Protagonist. However, Bandai and Toei didn't approve the idea.
Instead, the protagonist had to be the one opposing this Dark Hero. This resulted in the creation of the Knight-like Dukemon.
The two were the starting point.