The only USB-IF-approved method that I am aware of for charging while exchanging data is by using an accessory charging adapter as defined in the USB OTG spec. This method only applies to OTG devices, which have a micro-AB receptacle.
The "Y" cables that I have seen have two "A" (upstream) plugs, one that carries data and power and one that is advertised to provide up to an additional 500 mA to the downstream device. They don't have any special ability to support connecting for charging and data at the same time. These cables are not approved by the USB-IF so use them at your own risk.
A device will normally draw only the amount of current needed. It's similar to attaching a device to a power supply that is capable of providing many amps of current; the device's input impedance limits the amount of current the device draws.
