Multi stage network.

besides making sure that IP wise you can have both systems online at the same time i personally would design the system in a fashion that you can easily separate the two networks physically. so have one or two single cables that you can unplug and be sure that everything is separate. disconnecting physically is just simpler and more error proof than doing it through config

When this is done every system is different from each other depending on operational preferences.
If you only want it for maintenance then connecting the 2 networks together all the time is probably a bad thing. 

Without some network setup plugging the networks together means the sACN from both network will cross over.
sACN is multicast so it doesn't matter what your IP ranges are even if they are in different subnet's gateways will output a merge of both source if both consoles are patched on the same universes.

The safest way to do maintenance would be have a network port from each venue somewhere near your maintenance computer and then just plug into the correct venue depending on which one you are working on.

Thanks. I guess it's the easiest way. To have a virtual network (will be fiber for all kind of stuff between the rooms) that have a port coming out where I have my control room, and connect when needed.

It will be a all ETC gear stage (Colorsource spot/par/linear/AV40) but I want to have the options to send Concert setups for re patching and mode changing if someone have been messing around with the fixtures. (Happens to often) without needing to bring over a computer to the place. Sometimes I get calls and can't run off. would bee nice to just reset the rig to a default state over network.

i hope for you that whoever sets up the fibre network knows what they're doing... ;)

Me to! ;-)