Clearly, there are some weird rulings to it. I'm pretty sure the soul of the Dragonborn isn't a sort of recycled/reincarnated soul like the Avatar in ATLA. Completing the Thieves Guild questline binds your soul to Nocturnal, leaving it eternally bound to guarding her Twilight Sepulcher. Being a werewolf binds your soul to Hircine. Maybe something similar happens with Hermaeus Mora in the Dragonborn DLC like with Miraak? So do one of them get his/her soul if they die, or does the Dragonborn just spend eternities partying all up in Sovngarde while they squabble over it? Just something I've been curious about for a while, and I also tend to get curious about other peoples' input on the matter.