In legal field "merger" means merger of judgment of lower court into that of a superior court. when a judgment of lower court is set aside by the high court then it is high court's judgment which is effective and lower court's judgment is said to be merged in the judgment of high court. thus, doctrine of merger relates to one or same case but decided by heirarchy of courts or tribunals in appeal/revision/writ petition etc.
consolidation of cases is entirely different thing. it is a procedure under which two or more cases having same or identical cause of action are clubbed together for convenience of court as well as of parties and to avoid multiplicity of cases and conflicting decisions.