does hot and neutral wire produce less heat in a single conduit than in two separate conduits?
It depends on the conduit if it is a conductor or not.
In fact the heat due to magnetic field is not from the cable but on the conduit and it is produced by the eddy currents induced by the magnetic field in the metallic conduits.
That is why in case of metallic conduits cables are arranged in order to cancel the magnetic field.
It is true also for three-phase systems. It is also considered on cable trays.