I have both Stack Overflow & Electrical Engineering accounts. But, I found that only Stack Overflow account can be attached to CAREERS 2.0 account. Why it is not possible for Electrical Engineering?
From memory when you sign-up only one account can be entered and that is used to see if you qualify for access. However once your application has been accepted under "view & manage your profile..." the following appears for me when I click on the edit accounts button:
From there you can add any example answers from each site that appear under your profile. Note that I haven't used Careers 2.0 much so hopefully this answers your question and you didn't have something else in mind.