And by "at once", I mean any console can be used at any time by merely pressing a button or switch. So no plugging in power or AV cables. Right now I have 18 systems hooked up to my TV. I have a bunch more systems that aren't hooked up properly yet because of power or AV/RF issues, but this is what I have so far:
On AV:
Famicom Twin
Nuon
Dreamcast
Playstation 2
FM Towns Marty
NES
SNES
N64
Gamecube
Jaguar
SMS
Genesis/Sega CD/32x
Saturn
Panasonic FS-A1 (A MSX 2 computer)
On RF:
SG-1000 Mark II
Colecovision
Intellivision
Odyssey 2