All the major browsers (Internet Explorer, Chrome, Firefox) have tabbed browsing where you can open several websites, each on a separate tab in the same window. As each web site is on a different tab you can easily switch between the open tabs.
What, though, about the case where you want to see two websites at the same time? All you need to do is to drag one of the tabs outside of the browser window. If you have two monitors, just drag a tab onto the second monitor and it opens in a separate browser window. If you only have one monitor you’ll need to restore the browser window first.
When you are finished, just close the second browser window or drag the tab back into the first window.