back up

Short answer is "it can't". Typical master/backup configurations would use network nodes instead of local I/O ports so both consoles have full access to all parts of the rig.

you will need to output all universes over sACN or ArtNet to a dmx gateway or have a gadget II on the backup and dmx mergers.
so setup:
Console and backup PC in to a network switch. from that switch out to one ore more gateways that output dmx to dimmers and fixtures.
Have a look at ETC's Response gateways. they are good for this and you open for RDM in your system to.

In practice that means run ethernet cable from both the console and the backup PC to a standard $30 network switch, then run ethernet cable from that to the stage and if you are on a tight bugdet use something like a $169 artnet to DMX box www.amazon.com/.../B00TDGS6AS which outputs a single DMX universe. You can also get 2 and 4 port versions if you need more universes. I have a couple of these and they were easy to setup and use, there are other manufacturers ones out there that would do as well.

As an aside using ethernet cabling and artnet to DMX converters where required makes it a whole lot easier than running long DMX cables and a lot more robust. eg you could also put another standard network switch at the stage and run separate runs from that to individual artnet DMX convertors if you want.

Thank you to all who answered this. I thunk it wouldn't work. I have ETC nodes and currently using a switch so everything ready to hook up. I assume it means both consoles are basically mirrored this way so when one does something the other shows it. This is great and an easy do for me since I already have everything.
I also have plenty of touring grade network cable but in this case dmx is already run to the console so will just move the two outputs from the ION to the node and away I go.