If you want it all to be above board and legal then your talking alot mate! When you purchase and CAD CAM dongle its called a "seat" you pay for the rights to use that seat under licence. The software, training etc is all thrown in as part of the deal. So all of the money goes to the legal right for the seat and use of it. The software is just a tiny part of the overall cost even though it cost the company the most to develop it.
Anyway, you can get patch software for USB dongles that copy the coding from one dongle to another, Im not sure about serial dongles but will ask my mate. He works for Solid Solutions in West Midlands. (will PM you when i find out more)
You can get a split cable if i can remember, it just plugs into your dongle and splits it two ways. I'm sure its on ebay, or just do a search in google.
What software are you running? If you call a CAM seller then they could sort you out with a cheaper second seat if you upgrade. I'm guessing it will be inbetween £1k-£3k if you use something like CAM Works or top end software. You can get much cheaper versions if your only using basic turning/milling machines.
Mike