Multiple Devices synching to same iTunes library over Wifi

I have an iPhone 4s and and iPhone 4 that I would like to sync to the same iTunes library wirelessly on a Windows 7 machine. Unfortunately, this isn't playing nicely. Often iTunes will show error messages that it failed to connect to one of them, forcing the other to fail. It doesn't seem to work consistently. Does anyone else have this setup or problem?

Well, the iPhone 4 sits in my car charged when the car is powered on. My iPhone 4s only really gets charged when I sleep at night, but come to think of it, you might be on to something as I have been charging it during the day sometimes after coming into the house from the car.. I have a feeling there might be a conflict with timing as well sometimes. I am going to watch this more carefully.