Below my comments:-
1: Deployment of 2 RPD on same OBIEE instance?
You can't have 2 RPD's in online mode loaded in OBIEE 11g as now everything is managed by EM.
Where as in OBIEE 10g you could do that by creating 2 ODBC DSN's one for each mentioned in nqsconfig.ini file
[ REPOSITORY ]
Star = sample.rpd, DEFAULT;
SecondStar = SecondRepository.rpd;
follow this doc for details http://allaboutobiee.blogspot.com/2012/03/open-multiple-rpd-online-in-obiee-10g.html and http://gerardnico.com/wiki/dat/obiee/repository
2: Merging of 2 RPD with 2 different and independent subject area
3: Creating 2 OBIEE instances with same OBIEE installation and deploying 2 RPD
Nope we cannot create two instances with the same OBIEE installation.
Do let me know if you have questions.
Mark if helps.
I would beg to differ here. Multiple instances of OBIEE on the same computer, from the same installation are certainly possible. Just that we need to take the Software Only approach. For more information refer to Recommended Installation Scenarios for Oracle Business Intelligence - 11g Release 1 (11.1.1)