Sounds like a power issue. Most front USB ports connect to a single header on the motherboard, so there are 2 cables running from the USB ports to a single connector that plugs into the motherboard. Seems that there is enough power for the first device to get a data connection but not enough for both devices to get data connections when you plug in the second device. your ipod, etc is recognizing that it is getting enough power to charge but it doesn't take much for that to happen, the data connection requires more power.
You might be able to get new front USB ports and see if that fixes the issue.