How come 2 MOSFET chips are needed in protection circuits?
MOSFETs have a parasitic body diode which is an inherent part of their construction. If a single FET is used to protect the cell then (depending on which way around the FET is connected) the body diode will conduct either during charging or discharging. 

Placing two MOSFETs back to back prevents this undesired action, as one body diode will always be reverse biased no matter which direction the current tries to flow.