Apple only allows a device to be synced up with one computer at a time. Trying to sync it with a subsequent computer will result in the message that your wife got... all it's saying is that iTunes will wipe the content off of the device that was synced from the first computer and sync it with the content of the second.
Originally Posted by dfb8085
If your wife has the same content on both computers, go ahead and sync it with the computer that going to be generally more accessible.
Now, if, while you're in you home, some of the advanced features like home sharing might work for some devices (Apple TV or a different computer), it won't work with an iPod. iPad or iPhone... those devices can only play music that was synched to them, as far as I can tell. And even if it does work with those devices (don't think it will), when she leaves the range of your local wifi network, you will lose acces to the library. It MIGHT be possible to share the iTunes libraries across the network using the Home Sharing option and then sync the music from the laptop... but it's still going to be limited to syncing with a single computer - either the desktop or the laptop... or she will constantly be wiping/re-syncing every time she changes computer choices.
One option that may be work exploring is the iTunes Match, which basically allows you to pull individual music tracks down to your device from the cloud (music that you own/have previously synced with iCloud)... that'll cost you $25/year, but might be worth it.