You have two options in Y6 to get the extra two motor outputs needed. (1) Use the A0/A1 pins which are the SERVO1 and SERVO2 outputs. This is the most convenient since the EZ-connect cable already has these ready for ESC inputs. Or (2) if you are using PPM you can connect the other 2 motors to D5 and D6 which is labelled as P & Y on the RADIO connection.
Download the latest firmware package from the website for your board version. This will be configured for a quad, but you can then switch to Y6 in the config.h. If you are using mode 1 above, then you will need to define A0_A1_PIN_HEX in the config.h firmware file. You will lose the ability to do camera stabilization. If you use PPM, then you can still use SERVO1 and SERVO2 and camera stabilization.
There is no A12 on the Arduinos, there is D12, that is used for MODE 2 (AUX2), and A0 is the SERVO1 pin. I wouldn’t recommend doing it that way. Stick with the D5,D6 as the motor pins.
With PPM (SERIAL_SUM_PPM enabled) you will have all 4 AUX channels and you wont need the MODE 1 and MODE2 pins. Plug the PPM signal wire into the throttle input on the Quadrino. You’ll see all 4 AUX outputs work but, of course, you must have the mixes/switches programmed to the AUX1-4 positions on your transmitter. I find most transmitters are not configured for AUX modes by default and this is not something I can post here, you will have to refer to your manual.
Here is the Y6 pin config and the map between pins and the labels on the Quadrino.