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Draw a truth table and look at it from there. From your words, it is confusing. From your code it looks like this:

mainPin reedPin relayPin
HIGH LOW LOW
HIGH HIGH LOW
LOW LOW HIGH
LOW HIGH HIGH 

So:

void loop() {
 digitalWrite(relayPin(!digitalRead(mainPin));
}

If you want:

mainPin reedPin relayPin
HIGH LOW LOW
HIGH HIGH LOW
LOW LOW HIGH
LOW HIGH LOW 

then:

void loop() {
 digitalWrite(relayPin(!(digitalRead(mainPin) || digitalRead(reedPin)));
}

Draw a truth table and look at it from there. From your words, it is confusing. From your code it looks like this:

mainPin reedPin relayPin
HIGH LOW LOW
HIGH HIGH LOW
LOW LOW HIGH
LOW HIGH HIGH 

So:

void loop() {
 digitalWrite(relayPin, !digitalRead(mainPin));
}

If you want:

mainPin reedPin relayPin
HIGH LOW LOW
HIGH HIGH LOW
LOW LOW HIGH
LOW HIGH LOW 

then:

void loop() {
 digitalWrite(relayPin, !(digitalRead(mainPin) || digitalRead(reedPin)));
}