The night rune is very egotisical, cocky and selfish. Which I am guessing is the opposite of the sun rune? Since the opposite of night is day, so I do agree that even if it COULD connect to the sun rune again why would it? It has a will of its own, if the writers were to connect them, they would have to make it where the night rune was force to rejoin the sun rune, if not it would interfare with the night runes own personality and contradict it. And ofcourse, the Twilight/Dusk and Dawn were there to replace the night rune when it transformed itself into a sword and severed the bond, I think that's their role, nother more or nothing less.
Below is the information of Old Book Vol.2, you all probably know this already, but for the record I will copy and paste it here from the post that was made by Seldio in the S5 section of the forums.
Old Book Vol.2
The Sun Rune and the Night Rune
Long, long ago, the Sun Rune and the Night Rune were born into the world, their existence intertwined. They had a strong bond between them, as one controlled light, while the other reigned over darkness.
Before long, however, the Night Rune became annoyed by the dazzling
brilliance of the Sun Rune. The Night Rune could stand it no longer!
Turning itself into a sword, the Night Rune severed the bond between them, and went off on its own.
From the fragments of the bond left behind, the Dawn Rune and the Twilight Rune were born. Ever since the Sun Rune lost its other half, legend has it that these two new Runes have remained by its side, acting as guardians.
I just remember something, didn't the Dawn and Twilight/Dusk seem a bit similar to the Bright Shield and Dark Sword, with them fighting each other. It probably is just a coincidence, but I thought I would give it a little recognition.
Sandalphon - Senso Suikoden