Hi Sarah,
It’s hard to know exactly what you’re doing it without seeing it for ourselves, but I’ll give it a shot.
I’m guessing that when you say that you strike two strings at once, you mean that you strike the string you meant to strike and then you also graze the string directly below it.
If that’s the case, your issue would come from either hand position or thumb folding technique, although the latter is rather straight forward in most techniques. That means it’s probably just your hand position.
I would suggest trying two things, either together or separate. One, try bringing your wrist a bit closer to the strings. This will force your hand to change position so that when you place your thumb on a string, the string will be almost parallel to your thumb. Don’t push your wrist in too much, it’s a rather subtle change in position. Two, and I personally think this is probably a better solution, place your fingers as you would normally, and then twist your wrist towards the harp (that is, counterclockwise for your right hand and clockwise for your left)a bit and see if that helps. The overall effect will be the same, but the first proposed solution is more just to make sure that you’re not playing with your hand to far away from the strings.
Hope that helps!
Sam