One solution could be replacing your 2 GUI buttons to GUI Textures. Textures are placed in a Rect so in your update you have to check if your "touch" event is within one of these positions.
Have a look at: http://answers.unity3d.com/questions/380681/multiple-guibuttons-at-the-same-time.html http://answers.unity3d.com/questions/585943/multi-touch-gui-buttons.html
They might help!