Evolution of the semi-major axis (top) and eccentricity (middle) of two embedded planets. The simulation was continued from the isothermal model shown in Fig. 8 (red line) by adding an additional planet with the same mass at 2.0 AU. The green curve corresponds to the original model with the new planet added at t ≈ 2300 yrs. The red curves correspond to the (original) inner planet and the blue curves is for the (additional) outer planet. The bottom panel shows the period ratio (outer/inner) of the two planets.