Hey hey... the idea is not that bad actually.
Both voices can be used very differently.
Kristoffer and Mathias as two different characters as in Sceptre, or as two forces of nature or seasons (summer and winter).
Or a concept song with the subject of "conscience" or "bad conscience". One of them singing the "angel" part, the one who tries to keep evil away, who tries to appeal to reason. The other one singing the "devil" part, the one who is luring with ambivalent offers, who is trying to deceive and tempt.
Just a spontaneous idea that came to my mind right now...