There is absolutely no need to use flatpak.
In Mageia 9 two different packages "gimp" and "gimp3" are available and installable in parallel as they use two different installation paths. You have both applications in the application starter.
So to answer the original question: Yes, both versions can be installed and used at the same time from the official MGA 9 repositories. As "gimp3" is in reality Gimp 2.99.14 (a developement version on the way to real the Gimp 3.0) it is possible that there are also some small bugs. But in general it is stable to use. See
https://www.gimp.org/news/2022/11/18/gi ... -released/