This is a theory thats more a half asleep concept than anything, but I wanted to get it out here and in thoughts before it faded away. I entirely expect this to be quickly torn apart, but here it is.
First off, the Spiritual Realm, unlike the Cognitive and much unlike the physical, is if I recall correctly mentioned somewhere as 'All times, all places' which is why some tapping of it can see the future etr. So, when it comes to Spirit Realm, space isn't really an option/issue.
Second, in Elantris, there is a religion set known as Shu-Keseg, Shu-Korath, and Shu-Dereth. These effectively represented by my understanding, Unity through understanding, control, and love respectively. Also by my understanding, Shu-Korath represents Dominion while Shu-Dereth represents Devotion.
Now here is my thought. Dalinar has mentioned 'vision' like happenings including before bonding with the Stormfather. He has mentioned a similar feel, yet not the same. He has heard the voice of someone telling him to Unite them, which also has been seen by his brother. Some of these have again not been sent by the Stormfather. Once he does bond with the storm father, he gains spiritual adhesion. What if the Unity isn't Honor or anything related to that but instead somehow a mixing of Dominion and Devotion, which are two big parts of dalinars personality with Dominion being the first part of his life mixed with Devotion to his brother? It would be interesting to see if that was a hint with Shu-Keseg being the 'origin' of the two other religions that represent the shards, its focus being entirely unity before split into two factions.
Another note that might be to this effect is it would make sense it being two shards fused together as that would look closer to Adonalsium, which would explain Odiums panicked words of 'We killed you!' as he probably hasn't seen Harmony before/yet so this would be his first time running into such a being.
Having written this out, it seems far less plausible I admit, still I'm curious to see others thoughts on it.