const byte startPin = 2; // first button
const byte stopPin = 3; // second button

bool isRunning = false;

void setup() {
  pinMode(startPin, INPUT_PULLUP);
  pinMode(stopPin, INPUT_PULLUP);
  Serial.begin(115200); Serial.println();
  Serial.println(F("Ready. Press the start button (green)"));
  Serial.println(F("- Press the green button to start running"));
  Serial.println(F("- Press the red button to stop once running"));
}

void loop() {
  if (isRunning) { // we are running test if the secondary button is pressed
    if (digitalRead(stopPin) == LOW) {
      isRunning = false;
      Serial.println(F("Secondary button pressed, stop."));
    } else {
      // do what you need to do in a non blocking way when running

    }
  } else {
    if (digitalRead(startPin) == LOW) {
      isRunning = true;
      Serial.println(F("Primary button pressed, Running."));
    } else {
      // do what you need to do in a non blocking way when notrunning

    }
  }
}