dual console system

A lot more information is needed to give you an answer that will be realistic.
I have a system where I can have multiple DMX signals combined through an active DMX combiner to send one input to my dimmer racks while I feed DMX from a Net3 Gateway to the second input on my racks. I can also use a Gateway to take a DMX signal into my network.
This is dependent on:
What type of dimmers do you have?
Where are the consoles located?
Is there a lighting network in place?

So to answer your questions:
Yes, I have a similar type of set-up
Yes, an active DMX combiner. Doug Fleenor makes them. Or a ETCNet3 system with Gateways.
No, don't do the Y-cable.
Good luck,
John

A lot more information is needed to give you an answer that will be realistic.
I have a system where I can have multiple DMX signals combined through an active DMX combiner to send one input to my dimmer racks while I feed DMX from a Net3 Gateway to the second input on my racks. I can also use a Gateway to take a DMX signal into my network.
This is dependent on:
What type of dimmers do you have?
Where are the consoles located?
Is there a lighting network in place?

So to answer your questions:
Yes, I have a similar type of set-up
Yes, an active DMX combiner. Doug Fleenor makes them. Or a ETCNet3 system with Gateways.
No, don't do the Y-cable.
Good luck,
John

The simplest way to do this is the old A/B switch. Fleenor make those too.

Personally I would just swap show files on the Ion. I've taught high school students to do just that.One file for the show and one, with lots of subs, for events.