Splitting a series voltage to power seperate devices
In concept the 8x AA cells can nominally provide 12V as you suggest. You could tap into the middle of the stack and pull out the 6V level as well, The main thing I would say is to have the 12V device and the 6V device share the same GND connections back to the common point on the battery pack. This is important in particular because you may have your two devices integrated in some manner where circuitry has to share a common reference point.

Some things to think about when attempting to do this. 


AA batteries are limited in the maximum current flow that they can maintain. So make sure that the loads you have are well within what the battery can safely supply.
The AA cells do not have huge energy capacity and you will have to make sure that they can supply power to your devices for the intended working time.
The cells in the 6V part of the pack will be supplying current for both the 6V and the 12V devices so they will discharge sooner than the other four cells. This effect will be minimized of the 6V device draws a fraction of the power that the 12V device consumes.
12V and 6V levels being discussed are based upon AA battery cell technology that supplies 1.5V per cell. Be aware that there are various technologies that may have different cell voltages. Some of these technologies will do a better job than others at staying fairly close to their nomimal voltage levels as the battery is discharged. On others it will fall to a lower level sooner and may in fact be delivering current to your loads at less than 12V or 6V. You will have to study to check if your devices will continue to work properly when their feed voltage is lowered.