It sounds like there will not be that much to do this time of year. Just a few hours of looking at "Christmasy things" will be plenty.
So, maybe 1 week or less is enough time to see available tourist attractions that are viable in winter in both Seattle and Vancouver.
Maybe I will rent a car for a day or two in each city if the roads aren't icy or of it's not raining so hard that I can't see anything so I can do some sightseeing out of the city centers.
If the weather is clear enough for photos, it may be fun to take water taxis or ferries around and take pictures of the skylines for an hour or so.
I will try some Canadian foods and visit any interesting museums and aquariums and maybe walk through a large indoor shopping center to keep out of the weather, but I can't think of anything else to do to occupy time in those places in the winter.