Why does Oracle SQL*Plus consider two queries executed together on the command line as a syntax error?
It is a syntax error.

SQL*Plus expects either:


A single SQL command, terminated by either a ";" character or a "/" on a line by itself.
A PL/SQL block 
A SQL*Plus command


What you have entered is 2 queries on a single line, which SQL*Plus will send to the RDBMS - Oracle will then try and parse the string sent as a single query and fail because it is not valid SQL.

See the documentation for more details: http://docs.oracle.com/cd/E11882_01/server.112/e16604/ch_four.htm#i1039424