How does a shareable card work? Can my wife and I use the same card for 5 rides?
Yes, if it says "sharable" that's how it works. But if you're doing that you need to put the card through the reader twice.
After putting the card in the machine to release the turns turnstile for you (go through), you hand it back to your wife and she does the same thing. This page has more of a description:
http://www.barcelona-tourist-guide.com/en/transport/barcelona-metro.html#metroticket
Up to 10 people can use the same card. On a RS tour in Barcelona, our guide bought several and had us use them. We passed them back as we went thru the turnstile. I think it's cheaper to use the T10 vs individual tix.