I travel the route between Seattle and Bellingham about 5-6 times a year, I wouldn’t recommend business class. You get a $3 coupon and a slightly wider seat, but that’s it, and you pay a lot more money for it. If you get on in Everett, Stanwood or Mt. Vernon you may not initially get seats together when the train is 100% full, but a lot of people get off in Bellingham, and you’ll be able to snag seats together then. If you can’t get together initially, go to the club car and you can have a cup of coffee or whatnot and then you’ll be able to be together.
Also, lately the Seattle to Vancouver route hasn’t been on the Talgo trainsets for a while.