The two files need to have two different names in the F specs of your program.
Then you can do a OVRDBF before the call to the program.
You can also specify the library of the 2nd file within your RPG program.
REGISTER or login:
FQCUSTCDT1 if e disk extfile( 'QIWS/QCUSTCDT' )
FQCUSTCDT2 if e disk usropn
f extfile( CUSTCDT2 )
f extmbr( CDMBR )
f rename( CUSREC : CUSREC2 )
f prefix( QC : 2 )
D CUSTCDT2 s 21
D CDMBR c 'MYMBR'
CUSTCDT2 = %trimr( PgmLib ) + '/'
+ 'QCUSTCDT' ;