Is it safe to run two piggybagged Adafruit V2 motor shields on UNO with two different supply voltages? I want to drive some fans with the other shield with 12 V, and drop the voltage to 5 V with a buck converter for the other shield to drive some tiny DC motors. Will this work or will there be smoke?
Two identical shields stacked on an Arduino are likely to cause some issues.
They will compete for pin usage so at least one will need to be OFF BOARD and jumpered in a suitable manner to avoid both smoke and other issues.
Could you also take a few moments to Learn How To Use The Forum.
Other general help and troubleshooting advice can be found here.
It will help you get the best out of the forum in the future.
ballscrewbob:
Two identical shields stacked on an Arduino are likely to cause some issues.They will compete for pin usage so at least one will need to be OFF BOARD and jumpered in a suitable manner to avoid both smoke and other issues.
Could you also take a few moments to Learn How To Use The Forum.
Other general help and troubleshooting advice can be found here.
It will help you get the best out of the forum in the future.
Thanks. The shields themselves are fully stackable as per Overview | Adafruit Motor Shield V2 | Adafruit Learning System. It's just that I can't find info anywhere whether the two different supply voltages (or sources) to stacked shields would cause any issues.
Will also follow the forum etiquette in the future to the best of my abilities. 
The jumper connects motor supply (screw terminal) to V-in,
so if you leave the jumper(s) on, the 12volt fan supply will be shorted to the 5volt buck output.
Make sure you remove the jumpers.
Leo..