What you're talking about is often referred to as a GMPC (or DMPC). They can be made to work, but should be used with care.
In particular, there are two issues to be concerned with — spotlight and mental resources.
Spotlight is how much the attention gets to be on you. If your wife is playing one adventurer, and you're playing both another adventurer and the entire rest of the world, it gets really easy for your contribution to push hers out of the way. This is bad because it means that there's a decent chance that she'll end up out of the spotlight, feeling annoyed while you run large chunks of the adventure for yourself.
This is especially likely to go bad if you include any puzzle-solving scenes. Puzzles are always more difficult than they seem to be to the person who wrote them, and the temptation to "solve" the puzzle yourself after your player(s) have been stumbling around it for a while can get significant.
It's also bad because there's a good chance that you'll wind up mentally overworked. As the Lore Master, you have to come up with everything in the world that the party runs into. That's a lot of work to mentally keep track of, on top of all of the various characters that you'll have to play along the way because the player chose to interact with them. Adding in an additional Player Adventurer who can be expected to be on stage full-time is just making that harder.
So... if those possibilities concern you, what else can you do? Well, you could run the game with your wife as a lone adventurer. There are plenty of grand tales focused on lone adventurers. If you don't want that, I would suggest including friendly persistent NPCs only as secondary characters — those who can assist, but wouldn't be expected to solve puzzles, make decisions, or run the social interaction. One excellent one-on-one game I read at one point had the player's mule as an effective GMPC. A bodyguard, apprentice, servant, or whatever might also be appropriate, depending on the Player Adventurer in question. Basically, in combat and for problem-solving, they mostly follow orders, and they're a friendly NPC that the Player Adventurer can talk with at other times.
For more insight on the topic, I encourage you to take a look at other questions on this site in the gmpc tag. (The tag is also the link.)