Sending serial data in an interrupt
You cannot use Serial inside an interrupt. Transmitting Serial relies on interrupts being available, and from inside an interrupt they aren't.

All Serial communication must be done from loop().

So you need to just count the switch toggles and check to see if that value has changed in your loop.

volatile uint32_t toggles = 0;
uint32_t old_toggles = 0;
uint32_t ts = 0;

void setup() {
    pinMode(2, INPUT_PULLUP);
    pinMode(3, INPUT_PULLUP);
    attachInterrupt(0, toggle, RISING);
}

void loop() {
    if (toggles != old_toggles) {
        old_toggles = toggles;
        Serial.print("C1: ");
        Serial.println(toggles);
    }

    if (millis() - ts &gt;= 1000) {
        ts = millis();
        Serial.print("S1: ");
        Serial.println(digitalRead(3));
    }
}

void toggle() {
    toggles++;
}