For disc loading, the above is right. It is the rotor thrust divided by the entire rotor area. It may seem odd to include the hub and root cut-out areas but it is just a reference area and as long as you are consistent it doesn't matter.
As for mulitple rotors, I have always used the projected area. For co-axial rotors it is the same as a single rotor. For tandems I use the net projected area, ie dont include the overlap.
If blade loading is considered, you use the same approach. The rotor radius multiplied by the blade chord by the number of blades. I know this means some areas are covered more than once but as said above, as long as you're consistent it is okay. Plus, the inner parts of the blades/disc dont produce lift anyway.
Hope this helps, any more questions, drop me a line.
'Helicopters are like women, they're both complex things'
A.N.Modha, Rotorcraft Researcher
SEL