The Collaborative International Dictionary of English v.0.48:

Mutually \Mu"tu*al*ly\, adv.
   In a mutual manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mutually
    adv 1: in a mutual or shared manner; "the agreement was mutually
           satisfactory"; "the goals of the negotiators were not
           reciprocally exclusive" [syn: mutually, reciprocally]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "mutually":
   all at once, all together, as one, at once, au pair, by turns,
   collectively, commonly, communally, concurrently, conjointly,
   corporately, each to each, en masse, ensemble, entre nous, even,
   exchangeably, in a body, in agreement, in association, in common,
   in company, in concord, in conjunction, in exchange,
   in partnership, in return, in turn, in unison, inharmony, inter se,
   jointly, reciprocally, respectively, together, turn about,
   unitedly