Two fermions (particles that make up matter) cannot occupy the same
quantum state. If they are in the same orbital and have the same spin
then they will be occupying the same quantum state- which is not
possible because that would violate a fundamental property of matter.According to the Pauli exclusion principle, no two electrons can have
the same four quantum numbers. Two electrons in the same orbital will
not have the same four quantum numbers as long as they have opposite
spins, +1/2 and -1/2.