Yes, you can currently use a single Zune pass on up to six devices or PCs. However, Microsoft has announced they will be ending the $14.99/month subscription, and put in a $9.99/month which will rid of the 10 monthly song credits, and limit the number of devices to only four. This comes into effect only for new subscribers on October 3, so if the song credits and number of devices are important, I recommend purchasing a subscription now.
Also note that the Zune Pass only applies to a single Windows Live ID, so if your wife will be using a different ID, then a second Zune Pass will need to be purchased.
As for the software, it can link any number of Zune players or phones.