Arduino Multiplex RFID
There are silicon RF multiplexers available. It will be a much simpler prospect if you are using a lower frequency RFID, as it will be more tolerant to layout and mux-type components.

Here is an example of a DC-2.5GHz RF switch: (only an example, I know little of your application!)
http://www.analog.com/en/rfif-components/rf-switches/adg904/products/product.html

However it is likely, that the previous post you mentioned offers a simpler solution; using relays. RF relays are not too difficult to come by, but can be fairly expensive. (ie. perhaps more than a bunch of RFID readers...)