How to turn two LED's on and off with one button?
Just use the return value of digitalRead (which is 1 or 0) as the input to digitalWrite:

int btnVal = digitalRead(3);
digitalWrite(4, btnVal);      // On when button is HIGH
digitalWrite(5, 1 - btnVal);  // Off when button is HIGH


Or using boolean instead:

bool btnVal = digitalRead(3);
digitalWrite(4, btnVal);      // On when button is HIGH
digitalWrite(5, !btnVal);     // Off when button is HIGH