slowrotor,
As you know, the subject of autorotation encompasses numerous things, such as; rate of rotor deceleration, cleanliness of entry into autorotation, rate of descent, and usable rotational inertia at landing. However, I think you will find that there is no significant relationship between autorotational efficiency and rotor configuration.
Each configuration will have its nuances. There is a pitch-torque coupling on intermeshing helicopters that causes the craft to pitch upon loss of engine power. Twin-rotor craft do not have to consume some of their autorotative power driving the tail-rotor and its power-train. As mentioned by superiorheli, the coaxial and intermeshing helicopters take advantage of 'swirl-recovery'
There probably is a relationship between the number of blades and the rate of descent. However, this has to do with the fact that helicopters with more blades normally have greater disk loadings.
It's anybody's guess whether a 4-blade coaxial or a comparable 4-blade single will have the slower descent rate, but I will bet on the coaxial.
superiorheli,
Interestingly, the initial market for Charles Kaman's helicopters was crop dusting.
Unfortunately, this created a belief that the intermeshing configuration is low
disk loading and slow
speed .
Dave