In quantum mechanics, the concept you are referring to is called entanglement, where two or more particles become correlated in such a way that the state of one particle is intrinsically connected to the state of the other, regardless of the distance between them. This entanglement can lead to interesting phenomena, such as the situation you described where two entangled qubits always have opposite values.
If two entangled qubits are measured simultaneously, their measured values will still be correlated due to their entanglement. This means that if one qubit is found to have a certain value (e.g., "0"), the other qubit will be found to have the opposite value (e.g., "1"). Similarly, if the first qubit is measured to have the opposite value (e.g., "1"), the second qubit will be measured to have the opposite value (e.g., "0").
It's important to note that measuring a quantum system disturbs its state, causing it to "collapse" into a specific value. The simultaneous measurement of entangled qubits doesn't change the correlated nature of their values but rather reveals the values of the individual qubits at the moment of measurement. This phenomenon is often described as "spooky action at a distance" and has been experimentally verified in various quantum systems.
Entanglement and its effects are fundamental to quantum mechanics and play a crucial role in quantum information processing, quantum communication, and quantum computing. Researchers are actively studying and utilizing entanglement to develop new applications and understand the fundamental nature of quantum mechanics.