  Stellar mergers are one important path to highly magnetised stars. Mergers of
two low-mass white dwarfs may create up to every third hot subdwarf star. The
merging process is usually assumed to dramatically amplify magnetic fields.
However, so far only four highly magnetised hot subdwarf stars have been found,
suggesting a fraction of less than $1\%$.
  We present two high-resolution magnetohydrodynamical (MHD) simulations of the
merger of two helium white dwarfs in a binary system with the same total mass
of $0.6\,M_\odot$. We analyse one equal-mass merger with two $0.3\,M_\odot$
white dwarfs, and one unequal-mass merger with a $0.25\,M_\odot$ white dwarf
and a $0.35\,M_\odot$ white dwarf. We simulate the inspiral, merger, and
further evolution of the merger remnant for about $50$ rotations.
  We find efficient magnetic field amplification in both mergers via a
small-scale dynamo, reproducing previous results of stellar merger simulations.
The magnetic field saturates at similar strength for both simulations.
  We then identify a second phase of magnetic field amplification in both
merger remnants that happens on a timescale of several tens of rotational
periods of the merger remnant. This phase generates a large-scale ordered
azimuthal field. We identify it as a large-scale dynamo driven by the
magneto-rotational instability (MRI).
  Finally, we suggest that in the unequal-mass merger remnant, helium burning
will eventually start in a shell around a cold core. The convection zone this
generates will coincide with the region that contains most of the magnetic
energy, probably erasing the strong, ordered field. The equal-mass merger
remnant instead will probably ignite burning in the center, retaining its
ordered field. Therefore, the mass ratio of the initial merger could be the
selecting factor that decides if a merger remnant will stay highly magnetised
long after the merger.
