uint16_t uiPart1 = 0x0001;
uint16_t uiPart2 = 0x0203;

void setup() {
  Serial.begin(9600);
  // put your setup code here, to run once:

  uint32_t uiAssembled = ((uint32_t)uiPart1 << 16) | uiPart2;
  Serial.println(uiAssembled);
}

void loop() {
  // put your main code here, to run repeatedly:
  
}