I haven't yet ever checked into a national park without the DW being along. We therefore have two Senior Passes along - hers and mine. Either one will get us in at whatever discount offered for a single vehicle.
By the way, doesn't the "national park system" have a master database somewhere that matches the Senior Pass holder's name to the number on the Senior Pass? It seems like they should/would and I have always assumed that was the case - just like with all other types if ID with a number assigned to it.
If so, why do you ever have to buy another Senior Pass once you've bought one? If that's not the situation, then what is that number on the pass really for when push comes to shove ... such as preventing counterfeit cards from being sold all over the place? When I have to self-register at a public campground the form I fill out always requires the number off my Senior Pass.
The same person having to buy multiple Senior Passes for themselves doesn't smell right. I'll bet that when you loose one, there's some way to reapply for a replacement at way less than the old $10 fee or the upcoming $80 fee.