I think
It has some pros and some cons.. with a 24-round season, you get 12 home games (assuming no byes)
Assuming an even split of home games, that's only 6 games each for Wellington and Christchurch.. you'd have to re-think the model for season-ticket holders.
Perhaps as well as the "both venues" full season ticket, offer a "Wellington only" and a "Christchurch only" package at reduced rates - with the option to add-on games at the other home venue?
For instance, I'm 30 mins away from Wellington. If we had a shared side with Christchurch I'd but a Wellington-games ticket, but I'd definately be interested in adding-on a ticket for one of the Christchurch games (a mid-winter get-away for a weekend). I couldn't fly down to Christchurch for all the other home games though :-(
A set-up I'd like to see is having the team based in one of those cities (lets say 8 home games in Wellington), has a 2nd home nailed-down in the other (3 permanent home games in Christchurch) and the remaining home game on a rotating roster (in alternate years it's taken to Dunedin, New Plymouth, Napier & Palmerston North), or have a bidding system for it.
There is still the issue with the lights in Welly... There are only one or two more times a year the lights are permitted to be on... unless most welly games are held during the day you will run into problems.