+1 to this feature request. It’s really weird to have seemingly identical windows, including support for multiple tab groups, but only one of the windows allows left and right sidebars. If you want to use the file explorer to open an existing note in a non-primary window, you have to track down your primary Obsidian window (which may or may not have any sidebars open), open the note there, then drag-and-drop it to the appropriate tab pane in the non-primary window.
I’d imagine that implementing this capability may cause some hard trade-offs to be made regarding how this should work with the Workspaces feature. Should each app window behave as an independent Workspace container, should Workspaces support multi-window configurations (including per-window sidebar state), or should users be able to choose between global and per-window Workspaces on a per-Workspace basis? Hmm. I lean towards Workspaces being scoped only to individual app windows, but I could totally see valid arguments for the global multi-window workspace scenario too.