Federal regulations require only one (participating) party notification. The real gray area comes in when you cross state lines in a telephone conversation between a one party state and a two party state.Also there are some fed regulations regarding that as well.
Well...it depends. WA state is included in that list, but it is legal for a citizen to record his interaction with a police office performing his official duty without informing the officer (e.g. recording an arrest).Did the OP even read the article that he posted to the forum. Right in the article it told you in which states you had to inform all parties.
"In 12 states (California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania, and Washington) you're required to get the permission of all parties on the line before making any kind of recording.