Free: Contests & Raffles.
You can try and get an answer from the WDFW but be careful
I asked the same question a couple years ago here and the consensus was there is no regulation prohibiting it. I wanted a second gun with goose loads so I don't have to do the panic unload, reload when the random goose came over in a couple of the areas I used to hunt.
Just make sure that the unattended shotgun is secured in a fashion that it can't be knocked over (by dog, etc.)
Reminds me of NY with their magazine ban where it could not hold more than 10 rounds or something like that. People just started carrying 4-5 guns in their vehicle (AKA .... a NY reload).