For many decades, the textbooks on helicopter aerodynamics stated that coaxial rotors require up to 41% more induced power then two single rotors in isolation.
Blade element theory shows that a 4-blade single rotor requires 27% more total power [approx. 33% more induced power] then two single rotors in isolation.
From the above, it is apparent that the 4-blade main-rotor / tail-rotor configuration is more efficient then the two 2-blade coaxial rotor configuration. Perhaps this is a reason for Igor's asymmetrical main-rotor / tail-rotor being a preeminent configuration, at least in western helicopters.
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Interestingly, the latest textbook now shows that the coaxial rotor is actually much more efficient then previously assumed. Experiments show it to require only 16% more induced power then the two single rotors in isolation. Perhaps, this should have been obvious, since there appears to be little difference between a single 4-blade rotor and two closely spaced counter-rotating 2-blade rotors.
In other words, the coaxial configuration is now shown to be more efficient than the 4-blade single rotor/ tail rotor configuration, by approximately 10%; and this, by no coincident, is the power consumed by a tail rotor.
Perhaps this 'aerodynamic revision' will cause developers to reassess the advantages of symmetrical twin-rotor helicopters.
Perhaps the world's first production helicopter, the symmetrical Flettner FL-282 will reappear as symmetrical V-22s and others.
Edited to change 'induced' to 'total' in 2nd paragraph, thanks to Barannfin's question.