The Stereo Pulsar Variable Shape Tremolo Pedal from Electro-Harmonix is a tremolo pedal that allows significant control over the shape of modulating waveform. You need to hook up the Stereo Pulsar in stereo (each output to a separate amp/mixer channel) and you have a panner pedal that will send your instrument back and forth between two amps with the same control over waveform shape. Therefore, while one amp is fading up in volume the other will be fading down in volume, or while one amp is on, the other is off.

Stereo Pulsar Variable Shape Tremolo Pedal