To simplify switching between multiple Windows Live IDs you can now link them together, with the Linked Windows Live ID service. Instead of logging out and logging back in, once the accounts are linked, a simple click from the menu on the top right of most Windows Live pages will now do the trick. To link your accounts up, head over to the Linked Windows Live IDs section of the Windows Live Account page. Once they're all added, you must sign out and then sign back in.
However, there are two problems, one a potential security issue and the other more of an annoyance. The former is that it now only takes one password to break into multiple linked accounts. The latter is if one of your accounts is set to have you change your password regularly, or if one of the sites you visit requires a strong password, it will require that you change your password on each linked account.