SPI protocol with multiple devices

Question

I need to write and read the measurements received by a sensor on an SD card. The card reading would be to display the data on a TFT screen. Specifically, I am using the 1.8-inch ST7735 screen. To use the card with Arduino I am using an SD card reader module (ANGEEK brand). Both devices work with SPI protocol.

To verify that this would be possible I make a test code. In it, two functions (measure_V and measure_I) simulate the voltage and current measurements. So what I do is call those two functions to get 10 measurements and save all of them to SD. Then I read them and display them on the screen. I show them both on the serial monitor and on the TFT screen. In order to map the values ​​of V_data and I_data I use two functions that give me the maximum value in each measure. However, nothing appears either on the serial monitor or on the screen. The two devices share the MISO, MOSI, and SCK pins. Could someone help me with this? It is the first time that I use the SPI protocol with more than one device.

#include <Adafruit_GFX.h>
#include <Adafruit_ST7735.h>
#include <SD.h>
#include <SPI.h>

#define TFT_CS 10
#define A0_pin 8
#define RST_pin 9

#define SD_CS 7                                              // CS pin of SD card connected to pin 7 of Arduino

Adafruit_ST7735 tft = Adafruit_ST7735(TFT_CS, A0_pin, RST_pin);

const uint8_t rotation=2;                                    // Origin of coordinates top left

float V_data[10], I_data[10];                                // Store voltage and current measurements

File V_points;
File I_points;

const uint16_t  Display_Color_Green = 0x07E0;                // Green color


int measure_V(int x){
  int voltage;
  voltage = x;
  return voltage;
}

int measure_I(int x){
  int current;
  current = pow(x, 2);
  return current;
}

int max_V(){
  int maxVal = 0;
  for (int i=0; i<sizeof(V_data)/sizeof(float); i++) {
    if (maxVal < V_data[i]) {
      maxVal= V_data[i];
    }
  }
  return maxVal;
}

int max_I(){
  int maxVal = 0;
  for (int i=0; i<sizeof(I_data)/sizeof(float); i++) {
    if (maxVal < I_data[i]) {
      maxVal= I_data[i];
    }
  }
  return maxVal;
}

void setup() {

  Serial.begin(9600);
  tft.initR(INITR_GREENTAB);                                                // Initialize a ST7735R chip, green tab 
  tft.setSPISpeed(40000000);
  tft.setRotation(rotation);                                                // We set type of rotation
  tft.fillScreen(ST7735_BLACK);
  
  Serial.print("Initializing SD card...");

  if (!SD.begin(SD_CS))
  {
    Serial.println("SD initialization failed!");
    while (1) ;
  }
  Serial.println("SD initialization done.");
}

void loop() {

  int static n = 0;                                                              // Measurement number (initial measurement = 0)
  uint16_t pixelV, pixelI;

  // We perform and save on the memory card 10 measurements
  for(int i=0; i<=9; i++){
    V_points.print(measure_V(i));                                                // New voltage value
    I_points.print(measure_I(i));                                                // New current value
    }

  // We read the 10 measures of the card                                                              
  while (V_points.available() && I_points.available()){
    V_data[n] = V_points.parseFloat();                                           // Read voltage value from file
    I_data[n] = I_points.parseFloat();                                           // Read current value from file
    Serial.print("Voltage= ");
    Serial.print(V_data[n]);
    Serial.print(", ");
    Serial.print("Current= ");
    Serial.println(I_data[n]);
    pixelV = map(V_data[n], 0, max_V(), 0, 128);                                 // We map tension
    pixelI = map(I_data[n], 0, max_I(), 0, 160);                                 // We map current
    tft.drawPixel(pixelV, pixelI, Display_Color_Green);
    n++;
    }


}