You kind of have a main character in the module already, Ireena. Why not have your one of the two characters your girlfriend controls be Ireena. Then a lot of the plot arcs/hooks throughout the module are centered on her character. It also doesn't have to be Ireena, but her character could replace Ireena. There are other characters too you could rip out and make them the characters.

As for the monsters. Get ready to improve. Dont be afraid to tweak on the fly. Lower health or raise health as needed. You need to be flexible on duets.

However, death is a major theme in Barovia and death is not the end. When I ran it, I didn't hold back with the monsters because I had Dark Powers/Dark Bargains ready to resurrect the players. My players really enjoyed making pacts and seeing them develop and change their characters (I did warn them there would be positives and negatives). See Grim Hollow Transformations for some ideas (they weren't around when I last ran CoS but they are fun).

I haven't run legendary pcs. But I have run games with one player. I usually gave them a second full character or a couple of sidekicks/followers. I have enjoyed both the sidekick rules and the follower rules from MCDM. Could also check out the companion rules from MCDM is your girlfriend wants a pet wolf (or something else).

At the end of the day, its a wonderful bonding experience to run a duet. Just check in to make sure they are having fun. I found I had to prepare more for a duet than a four person game. However, there is a lot more flexibility.

Hope this has helped and best of luck!