  The dynamically assisted pair creation (Schwinger effect) is considered for
the superposition of two periodic electric fields acting a finite time
interval. We find a strong enhancement by orders of magnitude caused by a weak
field with a frequency being a multitude of the strong-field frequency. The
strong low-frequency field leads to shell structures which are lifted by the
weaker high-frequency field. The resonance type amplification refers to a new,
monotonously increasing mode, often hidden in some strong oscillatory transient
background which disappears during the smoothly switching off the background
fields, thus leaving a pronounced residual shell structure in phase space.
