You can not login with multiple accounts at the same time unless you do so with multiple browsers. There is no way to prevent that, except perhaps comparing IP addresses of those logged in, and preventing the same address from being used more than once. I don't think we'd want to prevent that. There is good reason to be able to login with mulitple account, particularly if you are an instructor viewing content as an instructor, and at the same time as a student.
Perhaps if you described the problem that is occuring, we could suggest a solution (perhaps). Provide a detailed description so we can reproduce it ourselves.
Also, double check the version of PHP. Version 4.3+ is required.