The Collaborative International Dictionary of English v.0.48:

mutualist \mutualist\ adj.
   One practising or advocating the doctrine of mutualism.

   Syn: interdependent, mutually beneficial.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mutualist
    adj 1: mutually dependent [syn: interdependent, mutualist,
           mutually beneficial]