Use of 2 SOVs in S/D valves

It depends on the operating principle of the specific valve. To pick an example, a valve might need to be positively driven open, and also positively driven closed; this could require two solenoid valves as pilots to drive the main valve.

Like mentioned in the previous reply it depends much on application

A) If You using Spring Diaphragm It increases Reliability,But rarely used

B) If Your using Double acting Actuator 02 Nos are required Open & Close Commands

Regards

Jose