A good video editor that I use a lot when making split screen vids is Filmora. However it is paid but i do think it comes with a trial version. There's a lot of split screen options/collage options and the layout/user interface is easy to understand
There's also a bunch of apps on Android that do video collages (I think Inshot is one of them), which you could also use cause most of them are free
Most other editors (Vegas, After Effects, DaVinci, etc.) however require some transform/scale tool knowledge before making collage videos
