Google offers multiple sign-in, which allows you to use more than one Google Account in the same browser session for a limited set of Google products. Now, Google Finance has joined the group -- with multiple sign-in, you simply switch between accounts using upper right hand navigation tool. The site will smoothly transition to your other account, allowing you to view other account portfolios with a click of mouse.
To activate multiple sign-in, click your username in upper right hand corner of any Google page, including Finance. Click "Google Account Settings" and you're taken to Google Profile page. Then click on "Edit" next to Multiple sign-in setting takes you to a page which explains how multiple sign-in works and allows you to enable this feature for your account.