You can find all sorts of information by doing a search in the white box at the top right of the main forum page, and on Google. It used to be that you could combine multiple NOCs, but that changed quite a while ago. The following was updated on 2012-03-22. Hope this helps.
Pg 22
http://www.cic.gc.ca/english/resources/manuals/op/op06b-eng.pdf
Eligible NOC Codes Stream: Applicants must have at least one year of continuous full-time or
equivalent paid work experience in at least one of the listed NOCs and not combined partial year
experience in multiple NOCs. Anticipated short breaks between jobs are acceptable. For example, if an
applicant is employed in one occupation for a 4 month contract and before the end of that contract, has
secured other employment that will begin shortly after the end of first contract, this break in continuity
would be acceptable. The occupation must be listed in the MI.