MILLI

( (IP-MAT (NP-SBJ (D-N Þetta-þessi))
	  (BEDI var-vera)
	  (PP (P milli-milli)
	      (NP (NS-G jóla-jól) (CONJ og-og) (N-G föstu-fasta)))
	  (. .-.)))

ÞAR Á 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)))))))))

No complement of MILLI

Second PP idomsonly P if milli takes no complement (see similar treatment of Á MEÐAN):

( (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))

ÞESS Á MILLI

	  (PP (NP-1 (PRO-G þess-það))
	      (P á-á)
	      (PP (P millum-milli)
		  (NP *ICH*-1)))

SÍN Á MILLI

	  (PP (NP-1 (PRO-G sín-sig))
	      (P á-á)
	      (PP (P milli-milli)
		  (NP *ICH*-1)))