The product Id is assigned when you create the app in the app hub. For WP7.X especially, most of what you put in the WMManifest.xml is over/re-written as part of the certification process.
The bigger issue you are likely to hit is that you can't load multiple apps on a device with the same ProductId so if you ever want to load them in the same emulator instance or on the same physical device you'll need to change one of them.