Why file sharing from mobile is not available when multiple-screen mode is enabled?

Currently, Mobile devices (Android/iOS) in multiple-screen mode can only support screen-mirroring.

Why is multiple-screen mode not available on Chromecast?

The Chromecast protocol (Google Cast) is natively designed for "One-to-One" mirroring only. To display multiple screens simultaneously on the ViewBoard, please use the vCastSender app instead.