MacOS – Full screen mode in two monitors

No fun, but the answer is no. Best you can do is choose which screen your app goes full screen on, which is a minor improvement on previously. It's tricky to accomplish due to the new screens workflow, an extended desktop onto a 2nd monitor is still just one 'screen', and you want it to display 2 different screens simultaeniously - a not unreasonable request, but one which is sadly unsupported at present.

You can change your default display to the external monitor by setting a boot argument. Note that this will disable the internal display of your machine. It is described as the opposite of clamshell mode, but this isn't quite accurate, since this works when the clam is closed as well:

sudo nvram boot-args="iog=0x0"

To clear the boot argument, either reset the NVRAM or execute the following. The former will clear all NVRAM contents, the latter will only clear the boot arguments variable:

sudo nvram -d boot-args