Connect to second Wifi network using dongle
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