It's dependent... Those early days in the revival were absolute hell, from Radahn recalling those tumultuous days in the Shunning-Grounds from Mohg's childhood, to Mohg recalling the agony of the rot eating Radahn from the inside. It took time before they finally constructed a mental barrier to keep the memories seperate. But still, every now and again, something is bound to slip through the cracks, whether by accident, or curiosity getting the better of one of them.
And yeah, there is definitely bound to be some comedic shenanigans. But not without its share of angst.
Between Mohg seeing how happy Radahn was in his childhood, being raised by a family that actually loved him and his siblings... And Radahn being privy to the uglier aspects of Mohg's youth.
Mohg seeing how Radahn idolized both Radagon and Godfrey - Godfrey especially, which unveils a sort of jealous side from Mohg. And with the way Miquella treated Mohg's body, it's like... Trying to erase HIM, and present Radahn as a replacement son. And it's just one of MANY things that really hurts.
Radahn... Hot-headed Radahn is somewhat humbled by what memories he picked up from Mohg. Seeing as Mohg was, himself, a son of Godfrey, Radahn finds some room to at least respect Mohg.
Buuut there's still some moment between the two.
Also Radahn finding out that Mohg is the brother of that old furry bastard with the tail.
And yeah, you'll be seeing a mixture of the two fighting styles. Imagine having a hail of meteors coated in bloodflame flung your way. A shockwave of purple spreading across the battlefield, ending with an explosion of red.
Not only are you fighting Mohg and Radahn. Your fighting Artorias and the Demon of Hatred.
... Did have this thought of Mohg taking up Euporia(Since Miquella lost the twinblade privilege), but I'm not sure about the fighting style.
... Maybe he starts utilizing some of Morgott's tricks. Conjuring forth spectral versions of Radahn's greatswords and his own trident. Summoning a rain of golden swords as he performs his bloodboon ritual.
As for the quest? Yeah it STARTS off with Mohg wanting to get Radahn's soul out of his body. The only problem is that Radahn has enough consciousness to where he isn't ready to go...
... That's where I'm stumped. Who knew Radahn better? Jerren? Or Freyja? Did Radahn want eternal war? Would he want control of Mohg's body, now that that's all that's left?
Regardless, I don't think Radahn would be contented to merely fade away. If he is to die, he will die fighting.
There's rivalry and conflict between the two. Resentment burns hot from Mohg, and Radahn's not having any of Mohg's sass. But there's a certain respect that Radahn holds, and Mohg, in time, might find Radahn as a companion of sorts.
What is the detriment, beyond keeping these two identities from bleeding into one another?