Hey, I'm kinda new to effects, and I was wondering what Matthias Jabs uses to get that double guitar sound?
And how much are they, and what's the best one to get?

yeah, recording 2 guitar tracks left and right is a neat thing to do. and you don't need effects. if anything, they use a little bit of reverb.
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In the studio version of the song it sounds like he doubles the guitar part. Probably recorded the same part twice - I think he does this because the second time the melody repeats it switches from in unison to a harmony solo, with two guitars playing a harmonized melody.

There are effects that can do this. They make harmonizers now that can do harmonies based on the key you're playing in.