The Collaborative International Dictionary of English v.0.48:

Coact \Co*act"\, v. t. [L. coactare, intens. fr. cogere,
   coactum, to force. See Cogent.]
   To force; to compel; to drive. [Obs.]
   [1913 Webster]

         The faith and service of Christ ought to be voluntary
         and not coacted.                         --Foxe.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Coact \Co*act"\, v. i. [Pref. co- + act, v. i.]
   To act together; to work in concert; to unite. [Obs.]
   [1913 Webster]

         But if I tell you how these two did coact. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coact
    v 1: act together, as of organisms