Split View suddenly not working
I used to be able to split view Pages and Preview together (I open a PDF to refer to as I use Pages). I suddenly can't do it. I've tried the Mission Control > Displays have separates Spaces, but it doesn't work. All I see when I search on google is like "Split View doesn't work on some apps." But it has worked until now, I don't understand what changed.
I didn't update anything recently. My mac is up to date and like I said I've been using it just fine, but now it's telling me it's 'Not Available in this Split View'. I can use split view for Pages and Preview, but just not with together. I don't know what changed.
Help
MacBook Pro 13″, macOS 10.15