So when you said in your first message that you wanted to overwrite the message, you didn't actually mean that since you now say you want to concatenate.
Concatenate those two together and put them in one string indicator. Your VI shows you already know how to concatenate strings.
I don't understand why you needed to ask the question.
This is one of those times where it would be appropriate to use a local variable. Even Bob said in message 3 that many "LabVIEW practicioners would say (Almost) never use a Local Variable'". He didn't actually say never.
Now I'm not sure why you have the laser 2 indicator inside the for loops since you are going overwrite the values every loop iteration. It will flash A B A C A D. Where A is the initial string and B C and D are the new strings that show up on every iteration. I would just move the local variable outside the loops so it would show A B C D. A to start and B C D .... on each iteration of the loops.
Right click the terminal. Create Locat Variable. Change it to Write mode if necessary. Wire the other source to that and delete the extra Laser 2 indicator.
Please take those tutorials. This is basic LabVIEW.