Apologies for the late reply to this, have you seen our experimental release of a Multi Device Service which is available here?
The tracking model will attempt to identify any hands within it's field of view and whilst its not possible to place a firm limit on the range of the Leap Motion Controller itself potentially you could build your application to disregard tracking data outside of your desired interaction zone.
This experimental release is a good place to get started and begin your testing but please note that this is a very experimental release that is unsupported and presented as-is.