For the same motor, i don’t think you’d see much difference in temperatures between running at 24 or 48v bus voltage. For the same torque, the motor windings will have the same current, whatever the supply voltage. There would be less current in your supply wiring though and in the DC parts of the ODrive board, but that should be a minor contributor to heat.
The main advantage of running a high bus voltage is torque bandwidth - being able to change torque in a millisecond for a high performance servo requires a bit of voltage headroom.
As to DC DC converters, i see no reason not to have one piggybacking off the other, if that’s cheaper/easier to do, and it may help prevent ground loop issues between two mains power supplies.