One Vim session can live only in one window that is managed by the system window manager. You cannot have multiple windows that talk to each other. You can split a single window manager window using ":split" and the like. Vim calls these "windows". You can have multiple tab pages, each of which holding multiple Vim windows, but all these tab pages must reside in a single system window-manager window.
Vim has a help file somewhere saying that Bram Moolenaar knows this is a problem and it's on the todo list.