I appreciate the help from everyone... it is clearer .
Not sure why this was hard for me to understand ...
But let me get this straight
Xehamort in KH1knew that he wanted Sora for some reason,
But he just ASSUMED that Sora and Riku would end up in the realm of dreams?
he became the portal that day, just in case?
During KH1, Xehanort only had inclinations of his plan, it was not set in stone. He knew about Riku and Sora via Kairi, and although not explicitly stated, potentially through his connection with Terra, where having possessed his body he would have discovered about the ritual he performed on Riku.
Also, as was previously stated, in Coded we can see the robed man ("Ansem") helping Riku get off the island by opening a portal for him.
You can ask yourself, why did Riku all of a sudden want to visit the outside worlds? Kairi states that "Riku's changed".
This isn't in stone, but it would be safe to speculate that just as Young Xehanort encountered the robed man, who set him on his appointed path, the robed man also had a chat with Riku.
We don't know exactly how the islands all of a sudden became plunged in darkness, but from the dark ball that appears to swallow up the island, we can assume it was the same technique used by Master Xehanort on the Land of Departure.
Further, in Chain of Memories we see Zexion telling Riku that it was his fault the island was destroyed.
What I'm getting at is that it was the robed man, or Xehanort reduced to a heart that set all these things in motion, including Riku's change in attitude. How else would someone like Riku know to say, "The door has opened Sora! We can go wherever we want now!" It sounds like he was promised freedom of travel by someone like the robed man, in exchange for opening the world's door.
It can be assumed that he did all this on purpose to awaken the innate ability of the keyblade within Riku. And knowing that the keyblade awakened on that day, he knew that they would come back in time at some point to relearn how to use the keyblade properly. And either or candidate, Sora or Riku was apt at that time for Xehanort's plan, as you can argue that the keyblade had awakened for both Sora and Riku at relatively the same time, which is why Yen Sid sent both Riku and Sora back to that fateful night together.
To neaten it up, robed guy, Xehanort, saw the keyblade awaken within Sora and potentially Riku in real time, so he knew that one day they might return to relearn the way to use the keyblade properly, as being a keyblade master himself, perhaps it was common practice.
And about how you say he "became" the portal. He is literally the portal the whole time he is the robed guy. The moment he cast his own body out and became just a floating heart, he was a portal for any of the Xehanorts to access. He didn't choose to all of a sudden become a portal at that specific time.