The linear combination of the (ψ3 = C1ψ1 + C2ψ2) is normally the third quantum state |ψ3>, which is described by a wave function ψ3 and will describes the actual benzene molecule (resonance hybrid). The coefficients C1 and C2 will specify the contributions of resonance structures (Kekule structures, for our case C1/C2 = 1, |С1|^2 = |С2|^2 = 0.5) in a real molecule of benzene (resonance hybrid), which can not be described by separate resonance structures. And now it is important to note that the real molecule of benzene it really real and unique, and can not have a discrete description, ie the measurements we will never ”see” the one Kekule molecule (resonance structure 1), then another (resonance structure 2), and this is directly contrary to the principle of quantum superposition. Moreover, adopted in theory of resonance Kekule structure, ie resonance structures are ideal structures that do not exist in reality, because they have all the bonds are equal, and this is despite the alternation of single and double C-C bonds, which in reality are different length. Therefore, the principle of quantum superposition (for resonance theory) is not executed, even for quantum states |ψ1> and |ψ2>, because as resonance structures is not reality, and any physical quantity (real) we can not measure.
If, however, as the resonance structures we take the real ”curved” Kekule structures with ties that have different lengths, then the resonance theory does not make sense, since the transition from one structure to the other will vary internuclear distance. But then the principle of quantum superposition is applicable to all three quantum states (for |ψ1> and |ψ2> it is obvious, and for |ψ3> will be the one ”twisted” Kekule structure, then the other, at 50 : 50). In fact, no change bond lengths have therefore chemistry is not applicable.
Especially notice, that resonance structures (Kekule structures with the same C-C bonds) is not electronic tautomers. Real benzene molecule is unique and no tautomerism exist. Incorrect represent benzene molecule as transitions between electronic tautomers (electrons in benzene does not migrate from one bond to another). There is a single benzene molecule, which has a constant electron distribution in time and which can not be represented by a discrete description. Therefore, it is accepted in resonance theory that every resonance structure contributes to the a real molecule (resonance hybrid) (usually chemists determined the contribution by the ”eye”). And the third quantum state |ψ3> describes the real benzene molecule, which represents something between a resonance structures (remember the contribution of each resonance structure to the real structure (resonance hybrid)).