[MaskedRiderAsakura]

Well we know from episode 1 that Daiji was scared to transform into Revice, possibly out of fear of letting his inner demon out and potentially hurting someone.

So perhaps Kamen Rider Evil is that, except instead of Revice where Ikki and Vice are split into two separate Riders, we have more of a Den-O situation going on where the inner demon runs the show until Daiji regains control (which would be represented by the other form depicted on Evil's belt).

I mean, Ikki and Daiji are basically polar opposites in the sense that one charges in without thinking and the other overthinks to the point of hesitation. So it makes sense that their Rider powers would manifest in opposite ways, where one is a 1-in-2 Rider and the other is a 2-in-1 rider.