John, I think your questions have been answered. If you still are unclear, PM and I will get you in touch with someone at WDFW who can better assist you.
CP is correct: your primary tag can be used on the second hunt. However, the reverse is not true: the second tag could not be used as a primary tag. You are allowed to use any legal weapon for the area you are hunting; you are not restricted by your primary tag's hunting method. Please note that some areas in the region may be "firearm restricted", which could limit legal hunting methods.
I have purchased second tags before online, but that may have changed.
Since this is a relatively rare occurence, you're likely to not get anyone very knowledgeable to help you at retailers.
Because the second tags are no longer relatively inexpensive, I would be inclined to wait before purchasing one but that's a decision only you can make. If you get a call at 8:00 p.m. one evening asking you to hunt the next morning, you would not have time to purchase the second tag.
Best of luck to you.