Dual Screen Mode

[retrorepair]

I couldn't see a feature request for this so I thought I'd open this.

Basically I'm looking to build a dual portrait screen ereader and being able to use Foliate in dual full screen mode would be great. I'm not sure if there's a way to enable screen spanning in a full screen mode?

I figure some desktop users may find this handy too since rotatable monitors are quite cheap these days.

Is this something that might be considered?