Reading Multiple MPU 9250 Sensors using TCA9548A Multiplexer
You need to adapt the example you're following to follow how the MPU library does its construction and setup.
Looking at the documentation for the MPU library you are using, you should change the definitions to:
 
MPU9250 mpu_0(Wire, 0x68);  // 0x69 if you pulled AD0 high

Then if you read further to the &quot;Common Setup Functions&quot; section, you'll see that this library's setup function has this behavior:

This function returns a positive value on a successful initialization and returns a negative value on an unsuccesful initialization

So you need to adapt that too:
Serial.print(F(&quot;MPU.1 detected?\t&quot;));
if (mpu_0.begin() &lt; 0) { // the examples assume zero is ok too
  Serial.println(F(&quot;No&quot;));
} else {
  Serial.println(F(&quot;Yes&quot;));
}

You'll save yourself some trouble if you use an array for your sensors too.
MPU9250 mpus[] = { MPU9250(Wire, 0x68), MPU9250(Wire, 0x68) }; 
const int num_mpus = sizeof(mpus)/sizeof(mpus[0]);

Then your initialization can be done with a loop:
for (int i = 0; i &lt; num_mpus; ++i) {
   tcaselect(i);
   if (mpus[i].begin() &lt; 0) {
     // ...
   }
 }

And you can do similar things for interacting with the sensor. e.g.:
for (int i = 0; i &lt; num_mpus; ++i) {
   tcaselect(i);
   mpus[i].readSensor();
   Serial.println(mpus[i].getAccelX_mss());
   // ...
 }