Utilizing a connected screen as the interface for current remote desktop session at that display's native resolution, rather than screen mirroring having the iPad screen show other information (or while using the iPad in split screen with other apps) would enrich the experience many fold - especially paired with an iOS compatible mouse.
Mocha RDP and Jump Desktop are two RDP apps which support screen extension in that manner (though Jump, due to a bug, currently doesn't support resolutions above 1080p).
I have moved your topic into the iOS feature request section and assigned it to our engineering department.
Although our various support queues will be monitored for emergencies, Devolutions' offices will be closed on Monday, May 20, 2019.
The feature has been added to our todo list.