  We study the phenomenon that pairs of supergravities can have identical
bosonic field content but different fermionic extensions. Such twin theories
are classified and shown to originate as truncations of a common theory with
more supersymmetry. Moreover, we discuss the possible gaugings and scalar
potentials of twin theories. This allows to pinpoint to which extent these
structures are determined by the purely bosonic structure of the underlying
Kac-Moody algebras and where supersymmetry comes to plays its role. As an
example, we analyze the gaugings of the six-dimensional N=(0,1) and N=(2,1)
theories with identical bosonic sector and explicitly work out their scalar
potentials. The discrepancy between the potentials finds a natural explanation
within maximal supergravity, in which both theories may be embedded.
