TeXStudio Embedded PDF viewer

2.6.6 is the latest TexStudio Version. At the top left of an open pdf view, there is an icon that shows a split window. Click on that and you'll get the side-by-side view you ask for.

In TexStudio "2.11.2" click on "Tools" > "View" or F7 to enable the pdf side window.

In 2.7.0, click on "View" and the click on "Show embedded PDF small".