I presume that the Yuan-Ti is using Double Attack in order to "make a slam attack against a defender and a non-defender as a single action"
Double Attack (standard, at-will) Poison
The yuan-ti anathema makes two slam attacks, each against a different target.
Each of the attacks made possible is a separate attack. This power grants action economy - do a pair of things that normally would each be standard actions, for the cost of a standard action. The power does not help work around defender marks.
This differs from a Blast or Burst which is both a single action and a single attack and so can be used to work around defender marks.