Not a bad idea,
the changes in certification are explained at the Direct Gov site.
Follow http://www.direct.gov.uk/en/MoneyTaxAndBenefits/BenefitsTaxCreditsAndOtherSupport/Illorinjured/DG_175850
to there for details.
You do not need one and your employer should not want one for less than a week, you write your own for more than 3 days, and just tell them you are ill for up to 3 days.
You can't have two. See
http://www.dwp.gov.uk/healthcare-professional/frequently-asked-questions/
Show it to one employer and let them write it down, then show it to the other.