The "multi" class driver takes care of that. It properly calls the SET and GET functions 
with the correct index. The code for that is in multi.cxx:105:

 device_driver(m_info->driver_input[i], CMD_GET,
               i - m_info->channel_offset_input[i],
               &m_info->var_input[i]);

The "channel_offset_input" and "channel_offset_output" store the first index of the 
channel in the overall ODB array (where inputs and outputs are staggered together), so 
the device_driver is always called with an index 0...n each for input and output, but 
with different commands CMD_GET and CMD_SET. You can take the mscbdev.cxx device driver 
as a working example.

Stefan