Mart,
To consider 'disk loading' you must first define 'disk area'. You are considering the 'disk area' of the coaxial to be the sum of the areas of the two disks. Some rotorcraft gurus use your definition for the coaxial, whereas other rotorcraft gurus define it as being the area of a single disk.
Here is information on this subject.
I take the latter for two reasons.
1/ The latter is more logical when all twin-rotor configurations are being considered.
The effective disk area for twin-rotor craft should be defined as the sum of the areas of the two rotors; with any overlapping area divided by 2. This means that for craft with equal diameter rotors the following approximations apply;
~ A side-by-side helicopter = 2*1000 = 2000 sq-ft.
~ A tandem helicopter = 2*1000 - 200 = 1800 sq-ft.
~ An interleaving helicopter = 2*1000 - 500 = 1500 sq-ft.
~ An intermeshing helicopter = 2*1000 - 700 = 1300 sq-ft.
~ A coaxial helicopter = 2*1000 - 1000 = 1000 sq-ft.
2/ The use of the latter can also be justified when moving away from 'Momentum Theory' and looking at the rotor performances using 'Blade Element Theory'.
By comparing;
~ a single rotor with 2 blades,
~ a pair of coaxial rotors with 2 blades each (total of 4 blades),
~ a single rotor with 4 blades,
it can be shown that the performance of coaxial rotors with a small gap is closer to the 4-blade single than the 2-blade single.
Dave