DBM V 3.0 Official Clarifications
Overlaps and rear elements of double based elements. Overlaps and supporting elements. (18/10/00)

An element in side edge to side edge contact with the rear element only of a DBE does NOT count as an overlap on the front rank element.	An element in side edge to side edge contact with a supporting element only does NOT count as an overlap on the front rank element.
Element A counts as a flank contact on front element Z (dying automatically if recoiled.) Element B has no bearing on the combat.

[The P. 21 sentence - "If an element which is double based or providing rear support to friends in frontal combat with an enemy front edge is contacted to flank or rear, this is treated as a contact with the flank or rear of the front edge." - only applies when the contact is by an enemy front edge.]