Bibenzimidazole nomenclature: Indicated H and locants
Your understanding of the numbering of the individual cyclic systems is correct. If we consider the following arbitrarily simplified separate compounds, the numbering is in accordance with your expectations; low locants are given first to the indicated hydrogen and then to the substituent according to the usual rule P-14.4.

 

In ring assemblies of two identical cyclic systems that are joined with a single bond, however, low locants are given first to the junction according to Rule P-28.2.1.


  P-28.2.1 Ring assemblies with a single bond junction
  
  (…)
  
  Each cyclic system is numbered in the traditional way, one with unprimed locants, the other with primed locants. Lowest possible locants must be used to denote the positions of attachment. These locants must be cited in preferred IUPAC names; they can be omitted in general nomenclature when no ambiguity would result.
  (…)


After that, indicated hydrogen is named and numbered according to P-28.2.3.

Therefore, the systematic name is 1​H,3'H-2,4'-bibenzimidazole and not 1​H,1'H-2,7'-bibenzimidazole since the locants 2,4' for the junction are lower than 2,7'.