  We numerically investigate vortex lattices in rotating two-component
Bose-Einstein condensates in which the two components have unequal atomic
masses and interact attractively with each other. For sufficiently strong
attraction, the system is found to exhibit exotic ground-state structures in a
harmonic trap, such as lattices having a square geometry or consisting of
two-quantum vortices. The obtained states satisfy the Feynman relation, and
they can be realized with current experimental techniques.
