  Despite remarkable progress in developing multifunctional materials,
spin-driven ferroelectrics featuring both spontaneous magnetization and
electric polarization are still rare. Among such ferromagnetic ferroelectrics
are conical spin spiral magnets with a simultaneous reversal of magnetization
and electric polarization that is still little understood. Such materials can
feature various multiferroic domains that complicates their study. Here we
study the multiferroic domains in ferromagnetic ferroelectric Mn$_{2}$GeO$_{4}$
using neutron diffraction, and show that it features a double-Q conical
magnetic structure that, apart from trivial 180 degree commensurate magnetic
domains, can be described by ferromagnetic and ferroelectric domains only. We
show unconventional magnetoelectric couplings such as the magnetic-field-driven
reversal of ferroelectric polarization with no change of spin-helicity, and
present a phenomenological theory that successfully explains the
magnetoelectric coupling. Our measurements establish Mn$_{2}$GeO$_{4}$ as a
conceptually simple multiferroic in which the magnetic-field-driven flop of
conical spin spirals leads to the simultaneous reversal of magnetization and
electric polarization.
