AFAIK what you are asking in not possible using DataPump or any other utility. You will have to import both tables and write code to achieve this
You can probably do something along the lines of what you want using external tables with the datapump driver -
create external table x as select * from a union select * from b;
copy the resulting file to new server
recreate external table defintion in remote database
ctas table you want as select * from external table.
Not sure if this has any real benefit of doing individual exports of the 2 tables and then manually bringing them together but it may some some space effort and time?
The initial part of this is something similar to this blog http://blog.mclaughlinsoftware.com/oracle-sql-programming/creating-an-external-table-that-uses-oracle-data-pump/ and you should be able to figure out the rest?
See what you think?
The only way I can think of doing it is with a view. Create a view to select the combined columns and then
create able as select from view;
The good news is that you can use data pump to export a view (or views) as a table, utilising the VIEWS_AS_TABLES parameter.
See Data Pump Export documentation.
The not so good news is that this is a new functionality in 12c and you will have to upgrade before you can take advantage of it.