1 x male micro USB to 2 x female
That's an excessively non-standard cable. But "Micro USB Host OTG Cable w/USB power" is what you want. You need a cable for OTG Host mode, and charging. That one will give you a micro male to plug into the tablet, a micro mini for charging, and a usb A female. You might need a usb a female to micro female adaptor.

Well, it might not work though. http://mehrvarz.github.com/usb-otg-power-nexus7/ points out that the Nexus 7, the plain vanilla, pure android and no manufacturer bs tablet, needs two driver/kernel patches for USB Host Mode AND Charging to work at the same time.