How can I see two tex-files in TeXstudio simultaneously?

On windows I managed to have two TeXstudio sessions in two different processes by modifying the program shortcut (I keep mine on the taskbar) with what Tim Hoffmann said: you have to right click on the shortcut, go to properties and then on the shortcut tab you have to add --start-always in the Target field.

The shortcut's Target field should look like this:

"C:\Program Files (x86)\TeXstudio\texstudio.exe" --start-always

Note that there is a space between the " and --! That is important!

Now all you have to do is start the program twice and work on two different files. I suspect that working on the same file twice will create problems with the auxiliary files so one should be a bit careful. I work on two different TeX files in the same directory but they have different names so there is no overlap.

I have no idea how to do this in Linux but I suspect something similar should be possible!

Split screen is supported in TeXstudio since update hg 5927 (0ce664cc1a7e) in 2016-03.

You can split the screen by right clicking on the file tabs and selecting the appropriate options.

Editing multiple files with split screen view

Split screen is now supported since 2.11.0. Use the options from the context menu of the tab.

Split screen is currently not supported.

You can use the option --start-always to open multiple instances of TXS. See the manual. Note: all instances save their settings to the same location when exiting, so the settings of the last close instance will persist.