Two GMs mean having different points of view on how to present challenges and dangers to the protagonist.
Also, I really like to subvert the standard structure of role-playing games, which usually have a master and many players.
The 2 GMs is great idea, I agree. But there is no rules or mechanics or tools for GMs to make it something unique. You can, just, play DnD with 2 GMs, but there is also no reason for that. And in "The Voice in the Wood" there is nothing what 2 GMs can make better then 1 GM.
Don't get me wrong, the game is very good and fun for the evening, thank you for that! This is just a question that comes after the game in my group.