the sale must abide by law in both states.

so that refers to waiting periods and (i think?) taxes (for example, for an ar15 orbother semiauto mag fed rifle, youll need to mail a money order or check to wa state).

a handgun needs to be picked up in your own state tho. so only shotguns and rifles are something you can do this trip.

and i dont think youll need both of you to be present simultaneously. shell need to drop it off at the FFL, let them know who its for (as specifically as possible, full name, DOB). you show up, do the transfer, wait the period.

like i said, since there are the extra steps to abide by both states laws, if an "assault weapon", the FFL needs to send whatever paperwork it is they do to your local LEO, wait for approval or wait ten days. IIRC.

its been a while since ive bought a gun, in or out of state so.... maybe best to wait for someone who can cite RCW verbatim lol