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A joint sponsor: two senators who were cosponsors of new legislation.
tr.v. (kō-spŏn′sər) co·spon·sored, co·spon·sor·ing, co·spon·sors
To act as a joint sponsor of: corporations that cosponsored a marathon.
(Commerce) a person, company, or other organization that is one of the two sponsors of an event or initiative