You can get 3 years PGWP after doing 2 back to back program, but it depends on luck also!!!
A student who take direct 2 years course may get 3 years or 2 years, don't know what are the criteria to get this. According 2 CIC if you study for 2 years you will get 3 years PGWP. But there is no clear definition about 1+1=2 years program. However chk this link you can find your answer-
http://www.cic.gc.ca/english/resources/manuals/bulletins/2010/ob194A.asp
According to this link the revised program criteria is-
Students holding a one-year graduate degree, diploma or certificate from a participating institution in Canada after having obtained, within the prior two years, a certificate, a diploma or degree from a participating institution in Canada may qualify for a three-year work permit. This exception also applies to students who have left Canada temporarily between programs of study.
List of Participating Institution-
http://www.cic.gc.ca/english/study/institutions/participants.asp#