Translation:The actors come out from behind the curtain on which there is a large butterfly.
A színészek, akiken egy nagy pillangó van, kijönnek a függöny mögül. A színészek, akiken egy nagy pillangó van, a függöny mögül jönnek ki.
Both are correct. Actually, the original sentence is a bit flawed.
"a mögül a függöny mögül" is more like "from behind THAT curtain", and not simply "the curtain".
I don't think the English needs "that" -- the relative clause is enough to determine it.
"Which curtain? - that curtain." // "Which curtain? - the curtain on which there is a big butterfly."
But not "Which curtain? - that curtain, on which there is a big butterfly.", unless it's just a "oh and by the way, there is a butterfly on it" rather than using the butterfly to indicate which curtain you mean.