I tend to have a ton of apps/windows open at any given time due to a combination of tasks often requiring a fairly wide array of apps, but also to reduce the friction of context switching to a minimum (even with a lightning fast SSD, closing one set of apps/documents and opening another takes time). So at any given time I probably have apps/windows open for a few different tasks open.
Virtual desktops get heavy usage from me, but (Cmd|Alt)-Tab switching being caged off per-desktop would actually pose a problem, because when I reach for an app with that shortcut I'm not actively thinking about which desktop it's on — I just want to go to it, wherever it happens to be, even if it's been intentionally placed on a particular virtual desktop.
So app-scoped Cmd-Tab works well for me, because the number of entries it has is always reasonable to tap through and it includes entries from all desktops on both screens.
Virtual desktops get heavy usage from me, but (Cmd|Alt)-Tab switching being caged off per-desktop would actually pose a problem, because when I reach for an app with that shortcut I'm not actively thinking about which desktop it's on — I just want to go to it, wherever it happens to be, even if it's been intentionally placed on a particular virtual desktop.
So app-scoped Cmd-Tab works well for me, because the number of entries it has is always reasonable to tap through and it includes entries from all desktops on both screens.