Was there more than one conjunction? There were a couple of things that made me think it was a recurrent event, but I'm not sure if it ever says there was more than one, or if it's just something I concluded.
- The elves seem to have arrived earlier than the humans.
- The "invention" of the Witchers was to counter a threat from monsters who had come through during the Conjunction, which implies that humans were already there and dominant when these monsters arrived.
No I think that there was only one.
Both Elves and Humans arrived to the "known lands" of the Witcher Universe, from the sea. Much like in the Lord of the Rings. That does not mean that they were originally from another "dimension". But it also does not mean that they were not. It is unclear.
Possibly they were on different parts of the Witcher Globe, and both of them happened to migrate to the same place, in different times, possibly because of technological advances, magic or just pure luck. Where they originally come from is a total mystery.
The invention of the Witchers is somewhat explained in the books. See, Humans do not live long, relatively to other races. But they multiply a lot. Slowly but surely Humans multiplied, and pretty much took over the world. This naturally got them in contact with more monsters. Monsters that actually lived far away from Humans, find themselves practically living among them, due to the Human's expansion. When Humans realized that the monsters hinder their expansion, or if you will "survival", they created the Witchers.
The Conjunction of the Spheres, I believe, happened about 2000 years before the events in the novels. Humans arrived on the Continent 400 years before the events in the novels. Elves arrived before that. If I remember correctly, there has been a disagreement about the time that the Elves arrived. The Elves claim that they arrived about 1000 years before the humans did. The Dwarves and Gnomes claim that they arrived about 200 years before the humans.
I gotta say I trust the Dwarves' and Gnomes' words more.