how do I add a second iPod Touch to my apple ID account?

When setting it up, use your Apple ID when it requests you sign in or create one.

Get started with iPhone, iPad, and iPod touch - Apple Support

Just note that if yo use the same ID you both device will have the same Calendar and Contacts and other things and both devices will get Messages sent to them.

I recommend that your wife create her own ID and that you set up Family Sharing to share iTunes and App Store purchases

Set up Family Sharing - Apple Support