The decision was made today during a meeting of leaders of the For Reforms movement and the United Front for a Worthy Future for Kyrgyzstan.
United Front member Omurbek Abdyrakmanov told RFE/RL the two groups also decided not drop their plans to launch mass rallies next month.
"[The two opposition movements] decided, first, to jointly carry out their political activities; secondly, not to participate in a coalition government led by [Prime Minister-designate Almaz] Atambaev; third, jointly to prepare a new draft constitution; and forth, jointly to organize indefinite protest rallies set for 11 April," Abdyrakmanov said.
Earlier today, President Kurmanbek Bakiev nominated Atambaev, the leader of the Social Democratic Party and a member of the new United Kyrgyzstan opposition movement, to be prime minister.
The move came after Azim Isabekov resigned, a day after Bakiev blocked his attempt to sack five ministers.
Parliament must now consider Atambaev's candidacy.