The Collaborative International Dictionary of English v.0.48:

Jointly \Joint"ly\, adv.
   In a joint manner; together; unitedly; in concert; not
   separately.
   [1913 Webster]

         Then jointly to the ground their knees they bow.
                                                  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

jointly
    adv 1: in collaboration or cooperation; "this paper was written
           jointly"
    2: in conjunction with; combined; "our salaries put together
       couldn't pay for the damage"; "we couldn't pay for the
       damages with all our salaries put together" [syn: jointly,
       collectively, conjointly, together with]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "jointly":
   all at once, all together, as one, as one man, at once,
   back to back, cheek by jowl, coactively, coefficiently,
   collectively, combinedly, commonly, communally, concertedly,
   concordantly, concurrently, conjointly, cooperatingly,
   cooperatively, corporately, each to each, en masse, ensemble,
   entre nous, hand in glove, hand in hand, harmoniously, in a body,
   in agreement, in association, in chorus, in common, in company,
   in concert with, in concord, in conjunction, in harmony with,
   in partnership, in unison, inharmony, inter se, mutually,
   respectively, shoulder to shoulder, together, unanimously,
   unitedly, with one accord, with one voice