In simple terms (cos I'm a pretty simple bloke!) the AS 350B has a single hydraulic system and the hydraulic accumulators allow the pilot to set the aircraft up in a suitable configuration to cruise and make a slow running landing in the event of failure of the hydraulic system. The fewer and the smaller the control inputs the pilot makes the longer the accumulator pressure will last.
The Twin Squirrel has a dual hydraulic system for the main rotor controls, but the tail roro is only powered by a single system. When the MAUW was increased by 140kg on the F2 model a hydraulic accumulator was added to the tail rotor control system to assist in maintaining yaw control in the event of a single hydraulic system failure on take off at masses in excess of 2400 kgs. There is a small hydraulic accumulator on top of a lateral servo jack (and when the pressure against this is increased beyond a set value it brings on a 'Limit' warning on the CWP).
The SA 365C2 has a dual hydraulic system, but again only a simplex tail rotor control system. The hydraulics are unusual in that one set of jacks is connected direct to the main rotor control system, whilst the other set is in tandem with it, but separated and under the floor. It too has a hydraulic accumulator system to warn of excessive load on a lateral servo. The SA 365N also has a simplex tail rotor control system and the same 'Limit' warning. The SA 365N1 and N2 models were the first to introduce duplex tail totor hydraulics and both have the lateral servo 'Limit' warning system.
Hope that helps.