Interesting post, from the author behind the book called the Long Tail, about his two network cables:
"On my desk at work I have two ethernet cables. One is black and one is white. The black one is connected to our corporate network. I use that one when I want to print things. I could also use it for Internet access and stuff, but I don't because the corporate network blocks a number of ports, including those used for Skype and Second Life. It's also pretty slow.
The white cable, meanwhile, is a standard consumer-grade DSL connection to the Internet, with nothing blocked at all. Our local IT staff installed it by popular demand, possibly without checking with headquarters (we love our local IT staff!). It's fast. I use it all the time."
Incidentally, I've certainly come across the concept of dual-LANs in an organisation - e.g. production and office networks that are kept separate.