That theory is fun, and it would work the other way around, with Toriko being the phenomenon. The amazingly beautiful girl, that have the perfect skills to survive in the wild, while also having experience with firearms... That's too perfect as well.
That would work well if .... the story wasn't told from Sorawo's perspective. She is the narrator, so that wouldn't make any sense. Also, she is also too coherent to be a phenomenon.
But I believe there are some truths to it. My own theory is that the otherside wants to establish proper contact with the human world. For that, they selected those two and it prepared them for that, making them lose their families so they wouldn't have any attached to the real world. The circumstances of Sorawo's family's death are too suspicious to begin with, as well as Toriko's moms dying on a plane crash.
The otherside also needed a couple with a strong bond, as solo human as not able to survive in the otherside. It also wanted someone with intellectual abilities (Sorawo) and someone with physical abilities (Toriko), in order for them to survive properly the physical and mental hardships of the otherside.
And for them to meet, the otherside used Satsuki as a catalyst, who was once a human, but was transformed into an otherside phenomenon. It is also possible that Satsuki had been selected in the first place to be the intellectual one to be paired with Toriko, and that she was deem unsuitable.
I don't believe the otherside to be an intelligent phenomenon that planned all of that properly. For me, the way it works is more like fate or destiny.