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

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "coact":
   accord, act in concert, act together, affiliate, agree, ally,
   amalgamate, associate, band, band together, be in cahoots,
   be in league, club together, coadunate, coalesce, coincide,
   collaborate, collude, combine, concert, concord, concur,
   confederate, conjoin, connive, consolidate, conspire, cooperate,
   correspond, do business with, dovetail, engage, federate, fuse,
   get heads together, get together, go partners, hang together,
   happen together, harmonize, hold together, hook up, interact,
   interchange, interlace, intermesh, interplay, intertwine,
   interweave, interwork, join, join in, join together, keep together,
   league, league together, make common cause, merge, mesh, mortise,
   partner, play ball, pull together, put heads together, reciprocate,
   stand together, synchronize, synergize, team up, throw in together,
   tie in, tie up, unite, unite efforts, work together