Looking for the voltages on the V+ and V- lines of an Apple 12W charger that corresponds to the 2.4A output.
The D+ and D- (I think you made a typo in your question) lines must be held to 2.7 V to support 12 W charging.
Unfortunately I don't have access to the Apple MFi Specification (which would be the best source), but this forum entry and subsequent answers by TI employees indicate that 2.7 V is the correct value.
Furthermore, this schematic below uses a resistor divider of 43.2k and 49.9k on the D+ and D- lines, which supports the TI forum responses of 2.7 V.
This is the schematic of the ACT4523A 5V/2.4A iPad Car Charger (Page 13 of this document, for your reference)
After taking some measurements, a 2.4A charger from apple applies approximately 2.68VDC to both D+ and D-. Most usb chargers advertise compatibility/charge capability with a simple resistive voltage divider on the data lines. Just calculate the required divider to provide that voltage from Vbus, I would take your own measurements though... And be aware of potential damage to the consumer.