In the old version of the Opera was the ability to insert on the side panel of the website. Thus it was possible to use two sites in split screen mode.
How to do the same in Opera 30?
To create an extension with is not suitable, as some sites (m.vk.com) are not allowed to access itself via : Refused to display 'https://m.vk.com/feed' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'.
I've had success using this for getting through X-Frame-Options to use iframes. There has been a web browser for the sidebar available for quite some time as well, which also works with websites that block frames.