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To convince a reader that, in your well-built world, Souls are real and it the components of reality necessary for two Souls to be "mates" are in existence, you have to take into account the target audience for your Romance novel.
This speaks to the topic of verisimilitude we keep returning to as a primary tool of the far-out science-fantasy writer.
For many decades, Romance publishers and writers, and readers, didn't consider "Romance Genre" as a science fiction genre -- and if "Fantasy" it was somewhere beneath bad comedy in the prestige list even though Romance has always out-sold Science Fiction.
Now, Paranormal Romance and Science Fiction Romance are considered "mixed" or "cross" genre.
My contention is that there is no mixing involved. Romance has always been science-fantasy.
Romance is "science" because it investigates the formation of bonds, just like chemical bonds, that we don't fully understand but we know they "just work."
Romance is "fantasy" because the plots represent the highest aspirations of the readers looking for a life-turning-point.
Most fans of Romance either know from experience or believe from self-knowledge, that Soul Mates
We either know a couple that just clicks like that, or we have been part of such a couple.
Those who flatly disbelieve in the HEA, the Happily Ever After ending, still enjoy a good Romance novel simply assuming that the ending is an HFN and eventually something will happen to catapult the couple into renewed misery.
As a Science-Fantasy subgenre, Romance has the opportunity to convey to the skeptics a real-world theory of what, exactly, Soul Mates are according to supernatural scientific theory.
Many religions grope into the problem of conveying a model of reality that includes Souls, immortal and otherwise, always trying to make it simple for the average person to grasp.
And where there are Souls, there is the possibility of two of them belonging together, somehow. Maybe it's unfinished karma from a previous life, or a Parent-Child relationship playing out, or some other theory.
Maybe, if you're doing Aliens on another planet, you'll need to invent one (probably more) religion that is substantially different from anything suited to humans. To do that in a way that human readers can grasp and learn from, you will need to know a lot about a lot of different religions and their take on Souls.
There are questions to ask.
1) How are Souls structured?
2) What kind of civilization would Aliens without Souls create?
3) What kind of civilization would Aliens with complete, whole, self-contained Souls create?
4) Can Soul Mates bond completely and still have one or both lack Happiness?
You'll need a real-world theory of Soul Mates, and a set of questions that probe your Alien culture to reveal how and why the Aliens differ from humans. Then you need to design a human who can bond across that Soul-Gap, and a reason why that human would do that.
Google Soulmates Explained and pull up a wide perspective on Souls and Mates. Keep the frame of reference we explored in "So What Exactly Is Happiness"
And keep asking yourself, as you read different philosophies, whether simply living with a Soul Mate, even bonding or marrying, produces Happiness.
Is meeting a Soul Mate a sufficient condition for the HEA? Is it even a necessary condition of Happiness.
Then check out this 2-minute video explaining Soul Mates:
Notice the glancing reference to "life's purpose" or the purpose of life, and what that has to do with happiness.
To convince the skeptic that the HEA is real, in fact common, there is a lot of thinking to do about what Happiness actually is, if it even is a real thing.
Find out what your target audience thinks happiness is, find out why they think that, then challenge the roots of that belief. Disturb your readers and you will engrave your byline on their memories.