Complex sentence: As they get deeper and deeper undercover they become addicted to the same heroin and amphetamines they're trying to keep off the street; they also fall in love, further complicating their assignment.
Split into simpler sentences: As they get deeper and deeper undercover, they become addicted to the same heroin and amphetamines they are trying to keep off the streets. They also fall in love, further complicating their assignment.