What version of Motosim EG/VRC are you using? I have not had any issues with the multi-window option on newer versions of EG/VRC. Currently running 2023 version.
I am currently using the 2022 SP1
Even though I have opened other CMOS files, and in those as well as the ones I generated in the classes I can use the multi window function, but now that I generated the file in this robot it does not allow me to use the function even though the tab does appear.