We discuss how simultaneous observations by multiple heliospheric imagers can
provide some important information about the azimuthal properties of Coronal
Mass Ejections (CMEs) in the heliosphere. We propose two simple models of CME
geometry that can be used to derive information about the azimuthal deflection
and the azimuthal expansion of CMEs from SECCHI/HI observations. We apply these
two models to four CMEs well-observed by both STEREO spacecraft during the year
2008. We find that in three cases, the joint STEREO-A and B observations are
consistent with CMEs moving radially outward. In some cases, we are able to
derive the azimuthal cross-section of the CME fronts, and we are able to
measure the deviation from self-similar evolution. The results from this
analysis show the importance of having multiple satellites dedicated to space
weather forecasting, for example in orbits at the Lagrangian L4 and L5 points.
