How to Expand 10-Bit ADC Signal for 16-Bit SPI Communication Between Two Arduino Unos

#include <SPI.h>
#define SS 10
#define MOSI 11

void setup() {
  pinMode(SS, OUTPUT);
  pinMode(MOSI, OUTPUT);
  SPI.beginTransaction(SPISettings(62500, LSBFIRST, SPI_MODE0));
}

void loop() {
  byte x = 0b00001101;  // Simulating 10-bit signal
  byte y = 0b11111111;  // Need to figure out how to pad or expand this
  
  digitalWrite(SS, LOW);
  SPI.transfer(x);
  SPI.transfer(y);
  digitalWrite(SS, HIGH);

  delay(1000);
}

#include <SPI.h>
#define SS 10
#define MOSI 11

void setup() {
  pinMode(SS, INPUT);
  pinMode(MOSI, INPUT);
}

void loop() {
  byte x = SPDR;
  byte y = SPDR;
  
  Serial.print(x, DEC);  // Converting received data to decimal
  Serial.print(y, DEC);

  delay(1000);
}