The Collaborative International Dictionary of English v.0.48:

Betwixt \Be*twixt"\, prep. [OE. betwix, bitwix, rarely bitwixt,
   AS. betweox, betweohs, betweoh, betw[imac]h; pref. be- by + a
   form fr. AS. tw[=a] two. See Between.]
   [1913 Webster]
   1. In the space which separates; between.
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            From betwixt two aged oaks.           --Milton.
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   2. From one to another of; mutually affecting.
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            There was some speech of marriage
            Betwixt myself and her.               --Shak.
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   Betwixt and between, in a midway position; so-so; neither
      one thing nor the other. [Colloq.]
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "betwixt and between":
   amid, amidships, amidst, among, amongst, between, betwixt, dull,
   fair, fair to middling, fairish, half-and-half, halfway,
   in medias res, in the mean, in the middle, indifferent, insipid,
   lackluster, medially, mediocre, medium, mediumly, mezzo-mezzo, mid,
   middling, midships, midst, midway, moderate, modest, namby-pamby,
   of a kind, of a sort, of sorts, passable, respectable, so-so,
   tedious, tolerable, vapid, wishy-washy