Keyblades have 2 different constitutions, either from the Light side or the Dark side. This is merely the exterior key, the inner key can act from either side and its abilities won't change. There is no clear division between good and evil. Usually the door to each world is closed and the keyhole itself is not visible. By bringing a keyblade nearby, the keyhole appears. But the lock is not actually closed itself. Indeed, the Heartless were able to find the door and easily open it and in this way steal hearts. In the previous game, Sora turned the locks and closed these doors. Furthermore, about the opened door of the integrated heart of worlds "Kingdom Hearts" itself, to prevent the outflow of the darkness within hearts the intent and reasons were different so the keyblade was used for a different purpose. In order to work, the door needs to be closed from both the Dark side and the Light side and both a dark and light realm keyblade are needed. The King needed a keyblade opposite from the realm of light keyblade that Sora had, so he went to the realm of darkness to obtain one.
The keyblade constructed from the Seven Princesses' hearts that Riku obtained in the previous game allowed for the keyhole in Hollow Bastion to appear. For the plan to cause darkness to overflow from hearts, the special hearts held by the Princesses were used to artificially make this keyblade. So it is different from a proper keyblade. There are further suggestions that this keyblade is artficial as well.
There isn't necessarily one keyblade for the dark side and one for the light side, just as many as there exist people with qualified hearts. Concerning the conditions to wield one, at this stage, "Those with strong hearts" is the only obvious one. However, less obvious conditions still exist and there are still plans for an opportunity to reveal those. Riku's "Way to the Dawn" and Kairi's keyblade are naturally the same type of keyblade as Sora's. However there is no particular explanation for the Soul Eater's transfer and occurrence, as well as Riku's handing it to Kairi. When there isn't a normal process of acquisition, I think its okay to think there is some deeper meaning there.
