I agree with everything Hawkshot99 said and just to add some voice of experience:
I assume this is your first bike from your description of only 400 miles?
If that is true, I would wait until you have a few thousand solo miles and a variety of riding experience under your belt before you venture two-up riding. That includes some twisty back roads, traffic riding, and just some highway cruising.
The dynamics of the bike will change a lot as Hawkshot noted. It will take more to stop it, a little more counter counter steering in a curve, and more control when you stop as you are hold up the extra weight.
Pillions can learn to ride two ways, one is to behave like a sack of potatoes and just sit on the back, in which you accomodate the extra weight. The second is teach them to lean in the curves too which is the easiest for both of you. You will just need to know how your extra weight you are carrying will respond in a consistent manner.
My wife goes with the sack of potatoes theory so I know thebikes response. I took my adult son out on the back of my ST1300 for a ride, and he was a new rider, and I did not even know he was back there in the curves. Big difference as I had never had a rider on the back before.
One other thing to realize is the pillion on a lot of bikes does not have the full view that you will as the pilot. They have to look around one side of your head or the other. If you see something of a warning on your right side and she is looking around the left, and you react, they might be a little jumpy as they will not see it :-)
If you wanted to practice some with her on the back, go to a large school or mall parking lot when it's empty, and practice there. Then you won't have to worry about cars, left turners, etc. But it would give you a variety of training for both of you in a safer environment, until you can work up to the street.
A lot of good advice on this forum as others will chime in too.
Another good forum I liked myself back when I just stared riding on the street is
A lot of good folks over there along with several MSF instructors and some good answers to questions like this.
Ride safe and glad your wife wants to join you.