They are both semi-transparent mirrors, but that’s where the similarity ends. The main difference is how well you can see through it. The coating on a teleprompter (beamsplitter) mirror is thinner, allowing more light to pass through. It’s more transparent and less opaque than a standard two way mirror, which is intended for security, not viewability. The coating of a teleprompter mirror is dielectric instead of metallic, which allows it to be color neutral (untinted).