I am running 10gage wire from a 40 amp breaker to a 40amp 2 breaker outdoor box. It will hold 2 20 amp breakers. 1 to run an air conditioner/heat pump (9000 BTU calling fror 15 amp breaker) and 1 for the wall outlets. The box will be mounted maybe 10 ft away. I was going to come off the 2 breakers with 12 ga romex. Do you see any problem here?

I don't know how long your run will be but that 10 awg wire is undersized for 40 amps even for a short run. Drop down to 8 awg or larger if run is more than 50'

Cool, thanks a lot.

The run is about 65', so I'll go with 8. will that fit on a 20amp single pole breaker?