I also faced a complicated matter with my reference letters. As I'm self-employed, I provided one letter from my accountant, stating hours, period, salary (income) and job title, along with supporting documents, but they couldn't testify about my duties. For the duties, I provided a letter from my father, who is my partner in the company, written on letterhead, stamped and all, supported by 4 other letters from third-party business partners of our company, stating that I had competence for some of the duties. The officer said that, although they have no problem with that kind of evidence, it wouldn't be accepted by CIC, as, besides only accepting letters from relatives under special circumstances, the letters with the duties weren't referred on the main letter, from my accountant.
So, the officer recommended that I should provide a sworn affidavit from at least one of the business partners, which should contain the necessary duties, and the affidavit must be referred on the letter from my accountant, as CIC wouldn't accept "fragmented" evidences. Right now, the letter and affidavits are being translated and shall be ready tomorrow. Even though I still didn't sent them, I have sent a sample with approximately the same contents as the ones I'm providing and she said that they will be accepted, as long as the duties cover the majority of the ones listed for my NOC.
My advice is to ask your nominee officer. If you provide your case number, they are usually fast to reply and very helpful. Just remember to be very polite!