Complex sentence: The double ranks of strings of the 12-string guitar produce a chorus effect, because the individual string sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one.
Split into simpler sentences: The double ranks of strings of the 12-string guitar produce a chorus effect. To produce this effect individual string sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one.