Can I use an RMAN Backup of DB1 to replace DB2
Sure you can, you can duplicate a database using RMAN backups. For example:


shut down DB2
delete all datafiles, redo logs, controlfiles of DB2
start DB2 in NOMOUNT
with RMAN, connect to DB1 as target, DB2 as auxiliary:

rman target sys/password@DB1 auxiliary /

duplicate the database until the required time, and rename files on the fly (if required), for example::

duplicate database to db2
until time "to_date('2016-02-20 16:00:00', 'YYYY-MM-DD HH24:MI:SS')"
db_file_name_convert (/oradata/DB1, /oradata/DB2)
logfile
'/oradata/DB2/redo01.log' SIZE 50M,
'/oradata/DB2/redo02.log' SIZE 50M,
'/oradata/DB2/redo03.log' SIZE 50M;