Instead of building the first text right away store the two numbers in variables and the operator in another variable. Then you can build the first text with number1&operator&number2. Then just add some conditions checking what the operator and set the second text depending on the operator.
global number number1=0
global number number2=0
global text operator=""
Start of layout
--- set number1 to round(random(1,100))
--- set number2 to round(random(1,100))
--- set operator to choose("+","-","/","*")
--- set Text1 to number1&operator&number2
operator = "+"
--- set text2 to number1+number2
operator = "-"
--- set text2 to number1-number2