Fuel system foibles:
AUX fuel is not available to the opposite side engine so after an engine failure, the fuel in the failed engine's AUX tank is dead weight.
Slipper fuel tank fuel IS available to the opposite engine via an electrical transfer pump to the same side main, then X-feed to the other side.
Excess fuel from the injection metering unit goes to the main tank on that side - no matter from where the fuel was originally supplied. 60 mins FBO out of the mains will allow for the return flow from full AUXs.
NB: Can't remember if there is an option for short or long range AUXs in the C402. There is in the C310. In which case you only need burn 30 mins out of the mains if the a/c has SHORT range AUXs.
So, if you're flying for range ie need the AUX & slipper fuel to do the trip, make damn sure you test the fuel transfer pumps from the slipper tanks (if fitted).
Then burn from the AUXs as soon as you can, allowing for space in the mains for returned fuel.
Then back to the mains, to make room for transferred fuel from the slipper tanks
Then transfer the slipper fuel. Each slipper holds about 60 mins of fuel.