I don't think that's possible. The protocol used for screen mirroring is a dedicated connection between two devices. I don't believe there's a way to add a second display.
I believe what you instead need is to make a stream available for display on multiple devices. One such way is to use VLC (a video player program that is available for almost any operating system) and use it to provide a multicast stream. The question then becomes how to display it on a Roku. There might be multiple ways, but what I would suggest is to use a DLNA server that can take the VLC multicast and stream it via DLNA, which any Roku device can display using Roku Media Player.
Dan
Roku Community Streaming Expert
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