Super Mario Galaxy 1 and 2

My wife and I have played through Super Mario Galaxy on the Wii twice and are almost done with our second playthrough of Super Mario Galaxy 2. They're both great games, but it makes me sad that I can't seem to find any other two-player games with a costar mode, where one player is capable of sustaining damage and does most of the controlling, but another player is immortal/invulnerable and can help out with certain things. My wife gets way too stressed out when she plays video games where she can die or sustain damage, but she loves helping me out on SMG. Anyone know of other fun video games with this mode? (I know it's also known, somewhat ickily, as "girlfriend mode", but whatever.) Our only console is a Wii, but I'd be into buying it on Steam, or possibly even investing in a previous-gen console, if there are enough good costar mode games for it.