From The Collaborative International Dictionary of English v.0.48 :
Unitedly \U*nit"ed*ly\, adv.
In an united manner. --Dryden.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
unitedly
adv 1: with cooperation and interchange; "we worked together on
the project" [syn: together, unitedly]