Your target/source text should be Test2, and your length should be based on your visible text Test, so you have those swapped. Also, your Test object needs to start out empty. That expression says set Text to whatever the length of Test was originally +1 character, so if your original text starts out already longer than your target text, you won't get the typewriter effect.
Update: So in the string do I swap Test1 and Test2 around? Also, I made the correct one empty, and put text in the other.