Connecting varistor and surge arrester in series
The combination of two MOVs in series with a surge arrester to earth offers you both common-mode and differential-mode surge suppression capabilities. A surge from line to neutral will be clamped by the series combination of the MOVs. A surge from either line or neutral to earth will be clamped by a single suppressor and the surge arrester to earth. The series MOV will also act to limit the current available to the arrester.

A single MOV (or two MOVs in series) works to clamp line-to-neutral surges but doesn't help you much for a line/neutral-to-earth surge. Splitting the MOV into two series elements with the arrester at the midpoint going to earth gives you the 'most' protection (at the expense of two extra parts.)

The series combination should be sized to provide the needed hold-off voltage (as you speculated, each could be half of the value of a single MOV) - then appropriately size the surge arrester (based on the series combination of one MOV + the arrester) in much the same way. 

Citing an EPCOS surge arrester note:


  Surge arresters must not be operated directly in power 
  supply networks. Because of the extremely low internal 
  resistance of these networks, an excessive current which 
  as a rule exceeds the permissible follow current would 
  flow through the ignited arrester. The arrester no longer 
  extinguishes and can reach very high temperatures.
  
  Varistors connected in series with the arrester are well 
  suited for limiting the follow current ... To stop 
  the arrester from responding during normal operation, a 
  permissible tolerance of the line voltage of +10% and a 
  possible derating of the arrester of –20% were taken into 
  account.