Anyway, Riku and Sora are best friends. They have been since they were little kids, back when they used to go to their secret cave together and draw graffiti on the rocks like good little children. Their relationship is then pretty much like any other best buddy duo. They go on adventures together and look out for one another. In the first game Sora has to literally knock Riku around until he realizes he's being controlled by Ansem. In the second, Riku has to leave hints for Sora to make him figure out what's going on with the evil Organization XIII. They even make a great fighting duo, as seen in the final battle. It's clear that they've known each other for a long time and trust one another.
At the end of Kingdom Hearts II, when everything is over and they're stuck wandering through darkness, they have a man-to-man talk. Both agree that they thought Riku was always better at things than Sora. Riku then admits that he was jealous of Sora for being so carefree. Sora retaliates saying that he wanted to be like the super cool Riku. And then, our lovely prince explains that he has one advantage having Sora for a friend. Aw, how touching!
As far as symbolism goes, Sora is the sky, and Riku is the land, making them opposites. The only time they meet is when the land happens to go high enough to reach the sky. Thinking about this in terms of the game, they rarely ever actually cross paths. Most of the game is with Sora running around to different worlds trying to find Riku, but he only gets to converse with him once in a blue moon. In that sense, the two are definitely connected, but they don't get to physically be near one another that much. They look out for each other from a distance, especially Riku.