I think he was getting at using a single 6V regulator. Input and output are connected as usual. 2 series resistors then go from the output to ground, with the junction of the 2 resistors being connected to the regulator 'ground' pin. There is a few problems with this (due to the ~5mA quiescent current of many regulators) that can be overcome by using small resistors (so that the current through the resistors is >>5mA, or else the error will be quite large).
The other way, which should have better regulation is to connect the 2 regulators that you have as follows: both inputs to the 13.8V, ground of reg 'A' to the ground. 'ground' of the reg 'B' to the output of reg 'A'. You should also connect a resistor from the output of reg 'A' to ground to pass the quiescent current (plus a little extra) of reg 'B', so maybe a 560 ohm (note that general purpose regulators can't sink current).
Note that the regulator you're using may have an headroom requirement of more than 13.8-12=1.8V.