Thanks Henra.
It is interesting to learn that the tandem's rotors need not be vertically separated due to the axis offset. Though I note this configuration would only be applicable within a very narrow range of situations, eg, limited to 2 or at most 3 bladed rotors, requiring a relatively larger longitudinal separation in order to minimise the intermesh, etc.
Having said that, I am still excited about this. Without the need for vertical separation of the two rotors in tandem, would this not make the tandem configuration viable for optimisation for speed? So, instead of the AVX's Defiant's coax with 2 fans for forward thrust, would a tandem (with both rotors intermeshing in the same plane) fitted with 2 forward thrust fans perform better for speed?