The Collaborative International Dictionary of English v.0.48:

Intercommunication \In`ter*com*mu`ni*ca"tion\, n.
   Mutual communication. --Owen.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intercommunication
    n 1: mutual communication; communication with each other; "they
         intercepted intercommunication between enemy ships"

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "intercommunication":
   ESP, agglomeration, agglutination, aggregation, alternation,
   answer, articulation, bond, bracketing, clustering, coaction,
   collegiality, combination, commerce, communication, communion,
   community, complementary distribution, concatenation, concourse,
   concurrence, confluence, congeries, conglomeration, congress,
   conjugation, conjunction, connection, contact, convergence,
   conversation, converse, cooperation, copulation, correspondence,
   coupling, dealing, dealings, engagement, exchange, fellowship,
   gathering, hookup, information, interaction, interchange,
   intercommunion, intercourse, interlacing, interlinking,
   intermeshing, interplay, intertwining, interweaving, interworking,
   joinder, joining, jointure, junction, knotting, liaison,
   linguistic intercourse, linkage, linking, marriage, meeting,
   merger, merging, mesh, meshing, message, pairing, quid pro quo,
   reply, response, seesaw, social activity, social intercourse,
   social relations, speaking, speech, speech circuit,
   speech situation, splice, symbiosis, talking, telepathy, tie,
   tie-in, tie-up, tit for tat, touch, traffic, truck,
   two-way communication, unification, union, yoking