Dual rail switching PSU critique
No, your feedback will not work. 

Your schematics has N-channel mosfets. They require gate to be more positive than source. In your design, when the system is on, you will have +24/+30v on source, and only 0/+12V on gate.
Even if you change mosfets to P-channels, you will still have at least 12v across gate-source when system is on is open. You would not be able to move to standby once you turn on your supply.



My recommendation is to place mosfets between ground and feedback pin (split R3/R7 into two series resistors, and use mosfet to short it)


Power requirements are simple to calculate:

I_input = ( I_output * V_output ) / V_input / efficiency


So assuming 85% efficiency, you will need 1.2 + 2.7 = 3.8A @ 12 volts.