Do we really need both tags?
I contend that we do not: I see only a few questions remotely related to solenoid valves (even accounting for those tagged with solenoid). Also, someone looking for solenoid valve information would probably look under solenoid as well.
I realize I'm not a high-rep or very active member here, so my opinion may not mean much. Thus, I want to know what the rest of the community here thinks about this.