The evolution of two-dimensional coronal magnetic arcades driven by photospheric shear flows is studied by numerical solution of the resistive MHD equations neglecting pressure and gravitational forces. By varying the distribution of the frozen-in photospheric magnetic flux, the shear flow profile and the magnetic Reynolds number, a fairly general picture is obtained. Isolated arcades develop in a quasi-selfsimilar stable way, invalidating previous studies of equilibrium sequences ▽2 ψ = αF(ψ) with monotonically increasing parameter α. Groups of several interacting arcades show a more complex behavior. When of sufficiently large height arcade structures tend to bifurcate, leading to plasmoid (or filament) formation. Usually this is a slow resistive process and the plasmoid is confined in the arcade interior. Configurations containing at least three arcades may give rise to fast plasmoid ejection.