You had the answer just not clearly the correct one, here are two windows at the same time that you should have got.
The key is your shortcut "Target" for the second instance should be
"whatever path to TeX studio\texstudio-2.12.14\texstudio.exe" --start-always
That is the unfamiliar double -- prefix at the start.
There are many places you may have a shortcut to start TeXstudio e.g. Start menu, taskbar, desktop or in my case a batch file.
The easiest way to keep an alternative in windows is create a copy from your existing start icon by right click and follow the link to existing folder or file location (usually where TeXstudio is located) then for that main item (shortcut or exe) right-click then
send to >
Desktop (create shortcut)
Right-click the properties of your duplicate desktop start-up (rename it on the
General Tab) and in the
target box go to the end and add a space then
--start-always similar to the example above.
The full range of options are currently listed here as ◦4.11 Synopsis of the TeXstudio command
For other ways to run dual compile using split editor and viewers see
https://tex.stackexchange.com/a/472480/170109 (split editor) and
https://tex.stackexchange.com/a/471069/170109 (split viewers) https://tex.stackexchange.com/a/472806/170109 (split editor and viewers)