You can create more than one account in Yahoo and you CAN log into all those accounts on the same laptop / computer. Before you can login to any of your other account, you’ll have to ‘Sign out’ of the one you are logged in to.
However, there is a way you can log in to different accounts on same computer at the same time. To achieve this, you need to have more than one browser, ie Internet Explorer, Google Chrome, Firefox, Netscape and other browsers, downloaded and installed on your computer / laptop. A session run on any browser is independent of sessions running on other browsers and hence this will allow you to simultaneously log in to more than one of your account at the same time.