The effect of the interactive flow on convective heat transfer from two vertical isothermal parallel plates have been studied numerically by the finite difference method. The Reynolds number, Grashof number, the relative length, L$_{2}$/L$_{1}$, and the dimensionless plate spacing, b/L$_{1}$ are varied as parameters. In case of outside mean Nusselt number, left outside mean Nusselt numbers show same values as L$_{2}$/L$_{1}$ and b/L$_{1}$ increase, but right outside mean Nusselt numbers decrease as L$_{2}$/L$_{1}$ increases. The inside mean Nusselt numbers are constant at narrow spacings and increase at wide spacings as Grashof numbers increase. The optimun plate spacing on left inside mean Nusselt numbers is b/L$_{1}$=0.4 at Re=100 and b/L$_{1}$=0.3 at Re=200. For the right inside mean Nusselt number, the optimum plate spacings move to the narrow spacing as Reynolds numbers increase and L$_{2}$/L$_{1}$ decrease.