Passing Value from Activity to Fragment

You can go many ways but given your current implementation (using a newInstance), I'd go with using your parent activity as a mediator, going like this:

1) Create a BaseFragment class which your TestFragmentOne and TestFragmentTwo will extend and in there hold a reference to your parent Activity (here named "MainActivity"):

abstract class BaseFragment : Fragment() {

     lateinit var ACTIVITY: MainActivity

     override fun onAttach(context: Context) {
         super.onAttach(context)
         ACTIVITY = context as MainActivity
     }
}

2) Then, in your Activity make sure you declare your variable as a field:

class MainActivity : AppCompatActivity() {

     var textVariable = "This to be read from the fragments"
     ...
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         textVariable = "I can also change this text"
         ...
     }
}

3) Then, from each fragment you can access your variable using the instance inherited from your BaseFragment:

 class TestFragmentOne : BaseFragment() {

      override fun onActivityCreated(savedInstanceState: Bundle?) {
          super.onActivityCreated(savedInstanceState)
          val incomingText = ACTIVITY.textVariable
          println("Incoming text: "+incomingText)

          // You can also set the value of this variable to be read from 
          // another fragment later
          ACTIVITY.textVariable = "Text set from TestFragmentOne"
      }
 }