Michale Spalt from Vienna, Austria is the owner of Spalt Instruments. Here are some pictures of what he calls "Hybrid" guitars. Double necks with a sustaniac element in the lower neck ( for i.e. "infinite" sustain or sounds comparable with an Ebow, a small device which is held above the snares) and a piëzo in the upper neck for simulating i.e. an acoustic guitar sound. And a picture of a double neck bass. See : http://www.spaltinstruments.com/
"BIG BEAT" -- In 1963 Jolana factory at Krnov started manufacturing a "wireless" guitar named BigBeat. Guitar of a very futuristic shape, as a matter of a fact a frame with a neck, it had a primitive vibrato and one pickup. A removeable amplifier was attached to the frame with special screws. It had 2W power rating and ran off four batteries. If you are bored with making music, it´s switchable to medium-wave radio (lol). Thanks to Gianluca Palumbo on Facebook for the info.