I have done similar things with both my wife (and others, to a lesser extent) over the years, where she’ll end up in a campaign she had helped me develop. Or a player will play through an adventure a second time, with a new table. We’ve found the line between DM and player is less stark than you may have been led to believe, and players in these situations can help in lots of ways.
I think the biggest challenge with co-DMing making sure both DM’s have something to do all the time, which can easily become the case for a co-DM who runs mechanics, certain NPC’s, etc.
At my table, we’ve tried having an “assistant DM” but have always ended up with a “player with an asterisk.”
My wife enjoys playing in the adventures that she helped create, just as many players (myself included) don’t mind playing through an adventure a second time. Having foreknowledge of an adventure takes away some of the suspense, but it can still be a lot of fun, just like reading a good book a second time brings is rewarding in different ways.
Of course these players should avoid spoiling the reveals for the other players. That is usually trickiest when they’ve forgotten a detail and the answer pops into their head. (Your brain loves to give you credit for “figuring things out” when you are actually just remembering them.)
Players with foreknowledge is nothing new. As a kid back in the 80’s, one of the players had invariably played or read through the published adventure — including that one kid who would read every single module in the store without buying them. “Don’t spoil it for the others” was the rule.
Potential Double-Whammy: GMPC + Significant Other
While my wife and I seem to have manage this well, there are big potential problems to avoid. DM’ing with a significant other as a player can often be problematic. A significant other that contributes to the creation of the game world just ups the ante a bit.
A potential disaster to avoid is for the game to become about your girlfriend’s player, with the other players “tagging along” like third-wheels on a date. Avoid even the appearance of this.
Unless you have buy-in from the other players, her character should neither be the center of the story, nor presciently optimized for the challenges that will unfold. That is to say, you want to avoid any unfair competition.
It can help if your GF can maintain a little bit of detachment from her PC. That is to say, she will want to find some of her fun in the way DM’s do, including by enjoying the game vicariously through the other players.
A second voice for narrative
That said, there is an opportunity for this character to play a special role in a fun way. Details about the story can be revealed through her PC, instead of the DM doing all the talking for this.
This sort of talk can happen in a published campaign setting (e.g., The Sword Coast) when some players have more knowledge of the setting than others. And lots of DM’s dip their toes into this sort of play by handing one player a note that they in turn reveal to the others.
In your case, you’ll probably want some believable reason why your co-developer’s character knows so much more stuff. (For D&D 5e, maybe consider the Sage background.)
If the players’ knowledge gives her character an advantage over the others that can be annoying. If it just lets her be good “color commentary” on the storytelling, that should just add to the play experience.
Here’s a trick that might not be for beginners, but such a player can provide hints to the party when otherwise the DM would need to. Done well, the rest of the party won’t even know they effectively got a “DM hint.”
Assistant to the DM, or DM’ing from the player’s seat
Foreknowledge of the story can help in littler ways too. Something as simple as a player being on the right page of the rulebook ahead of time can speed up play.
Any player can help DM/help the DM by keeping track of details, looking up rules, etc. This can be a way to stay engaged while you are avoiding spilling spoilers.