But in the second line I want to show the resistance at the same time and I have no idea how to manage this...

"No idea"? michinyon already explained the math

The analog voltage v is going to be 5 volts x ( 10k / ( 10k + R ) )

where R is the value of the thermistor resistance at the particular time.

Therefore R = ( (10k x 5 volts) / analog voltage ) - 10k

So now you just have to get the analog voltage and do the math.

Option #1: The simplest (but not most accurate method) would be to replace "analog voltage" with another analogRead.

Then you just do the math in code exactly like it is written:

float R = ((10000.0 * 5.0) / analogRead(0)) - 10000.0;

Now you can add some print code to the PrintTemp() function.

Option #2: Move the anlaogrRead() out of the function Thermister() and then call Thermister

I would change PrintTemp() to be this way, instead of as-written:

double fTemp;

int thermValue = analogRead(0); // get one analogReading

double temp = Thermister(thermValue); // Read sensor with the stable reading

float R = ((10000.0 * 5.0) / analogRead(0)) - 10000.0; // calculate resistance value

// later on inside of the function, add the code to print the value R.