形容詞 head to head:

1. involving two persons; intimately private
   同義詞：tete-a-tete, head-to-head
2. inconclusive as to outcome; close or just even in a race or comparison or competition
   同義詞：neck and neck, head-to-head, nip and tuck

副詞 head to head:

1. even or close in a race or competition or comparison
   同義詞：neck and neck, head-to-head, nip and tuck