You are supposed to notify Google that you have multiple accounts. They will then insure that you can't get double-serving on the same results page, which is not allowed.
You probably should also open a My Client Center account, which will allow you to select from your accounts on one page.
If you are advertising on behalf of different clients, then I don't think the rule above applies, but you probably want your clients to open their own accounts. Then you can manage them from your My Client Center account.
