Both the former chief ministers will share stage while campaigning for by-elections to 10 assembly seats to be held on August 21.
Nitish Kumar's Janata Dal (United) is now partnering with Lalu Yadav's Rashtriya Janata Dal and the Congress.
The RJD and Janata Dal will contest four assembly seats each and the Congress will contest two. The JD(U) rules Bihar with a depleted strength after the split last year with the BJP engineered by Nitish Kumar, who stepped down as chief minister after the disastrous showing in the Lok Sabha elections in May.
The parties finalised their alliance for the by-elections overe the weekend. JD(U) state president Vashisht Narain Singh called it "a new beginning in Bihar to defeat communal forces."
Last month, the RJD supported the JD(U)'s two candidates in the Rajya Sabha by-polls.