MILLI

In þar á milli, the complement of milli, that is þar, moves to Spec position of the higher PP:

 (IP-SUB (NP-SBJ *con*)
	 (VBDI fór-fara)
	 (PP (ADVP-1 (ADV þar-þar))
              (P á-á)
	      (PP (P milli-milli) (ADVP *ICH*-1)))))))))

Second PP idomsonly P if milli takes no complement:

( (IP-MAT (CONJ og-og)
	  (VBDI fór-fara)
	  (NP-SBJ (NPR-N Sigríður-sigríður))
	  (PP (P á-á)
	      (PP (P milli-milli)))
	  (. ,-,)))

If there were only one P involved, then the PP would be parsed as postpositional, parallel to "darüber" in German or "there about" in the English corpora as below:

 (PP (ADV there) (P aboute))