multi device usb support for beaglebone black
I have tried supplying power through external hub by connecting BBB's
  data pins and ground only. i think both devices get enough power but
  BBB doesnt support multiple usb devices.only one device works at a
  time


The BBB does support multiple devices using a usb hub and multiple users have reported success with of the shelf solutions.  It would seem to be more an issue that you are not using a hub that knows it is externally powered. During enumeration, all USB devices report how much power they need and if they are self powered. If a controller sees a load greater than what it can supply,  It won't activate some devices. Before giving up on the BBB, try it with an off the shelf powered USB hub. The hub's firmware has to know it is self powered, simply cutting to Vdd line is not enough.