Connect to second Wifi network using dongle I have a Macbook Pro running Yosemite (10.10.1) and an Asus wifi dongle.

I would like to connect to two wifi networks. One is a normal home/office/hotel/conference network with a connection to the outside internet, the other is a local internet with two other machines on it (a windows tablet and a PLC controller).

How do I do this? 

(I can't even tell if the wifi dongle is being recognized, but it's supposed to support Mac. When I use it with the windows tablet, I get a "Wifi" and a "Wifi 2" option, and I just set each to different networks)
If you have the USB-AC51 I am afraid it does not work with Yosemite.

In the specs it only shows support up to 10.8 and not for 10.10

source: http://www.asus.com/us/Networking/USBAC51/specifications/

It turns out that someone is claiming that the "Bearextender driver software" works for that, but I can not confirm since I do not have your hardware to test.

http://store.bearextender.com/pages/regarding-yosemite-compatiblity