Sharing iTunes downloads with my wife?

You can only download a song once, you cannot download the same song on two different computers.

You can copy purchased songs to another computer, and they will play there is you authorise the computer with your iTunes account.

You can also set up Home Sharing, which will automatically copy purchased songs from one computer to the other. Use iTunes Help for more info on Home Sharing.

Yeah, what Ian said is spot on.

Assuming you have a network both of your computers are connected to (like a wireless router for example), Home Sharing that Ian mentioned is probably the best solution.

You can learn more here: http://support.apple.com/kb/ht3819

Ultimately what this lets you do is once you login to one master account on both computers, you can then look at your wife's library assuming their computer is on and connected to iTunes. From there, you can either stream music, or drag and drop music from her computer to your own so you have a copy. You can also set up an automated system as Ian mentioned, where whenever one computer gets a purchase from the iTunes store, it will go to the other computer as well when you are both connected.

As for EARLIER downloads, if they are a few years old (when they were DRM protected), you will be prompted to enter the other person's login info. If they are DRM free, they'll play fine.