Logically Horizontale-Partitioned Tables
The answer is simple, as long as you can guarantee that self-joining the merged table will not affect all queries that hit the table.

Just merge the tables, then create a synonym for the old table names that points to the newly merged table.

eg:

create table mergedtable as ( select * from tablea union all select * from tableb );

drop table tablea;

drop table tableb;

create synonym tablea for mergedtable;

create synonym tableb for mergedtable;


The synonyms will allow the legacy code to "see" the new table as if it is the old one.