Evolution of the eccentricity (y-axis) and semi-major axis (x-axis, in AU) of a set of test particles under the influence of two planets without gas drag. Particles are coloured according to their initial location: red for particles belonging to the feeding zone of the innermost planet, blue for particles belonging to the feeding zone of the outermost planet, and green for particles belonging to both feeding zones. The two 10 M_⊕ planets are located at 5 AU and 5.6 AU. The first panel shows the initial conditions, the second panel is the state after 600 orbital times of the innermost planet, and the last panel is the state after 1200 orbital times of the innermost planet. As can be seen, test particles are very efficiently mixed in the combined feeding zone on a short timescale. Only planetesimals located in co-rotation resonance with one of the two planets are scattered on a longer timescale.