  The Swift mission has discovered an intriguing feature of Gamma-Ray Burst
(GRBs) afterglows, a phase of shallow decline of the flux in the X-ray and
optical lightcurves. This behaviour is typically attributed to energy injection
into the burst ejecta. At some point this phase ends, resulting in a break in
the lightcurve, which is commonly interpreted as the cessation of the energy
injection. In a few cases, however, while breaks in the X-ray lightcurve are
observed, optical emission continues its slow flux decline. This behaviour
suggests a more complex scenario. In this paper, we present a model that
invokes a double component outflow, in which narrowly collimated ejecta are
responsible for the X-ray emission while a broad outflow is responsible for the
optical emission. The narrow component can produce a jet break in the X-ray
lightcurve at relatively early times, while the optical emission does not break
due to its lower degree of collimation. In our model both components are
subject to energy injection for the whole duration of the follow-up
observations. We apply this model to GRBs with chromatic breaks, and we show
how it might change the interpretation of the GRBs canonical lightcurve. We
also study our model from a theoretical point of view, investigating the
possible configurations of frequencies and the values of GRB physical
parameters allowed in our model.
