The Asus Twinview Dock is an accessory for Asus ROG phones, it is an independently controlled external monitor that you slot your phone into, so that it resembles the dual-screen clam-shell configuration of a Nintendo DS or 3DS.
Currently, the Citra app does not recognise the dock as an external monitor, and no instance of Citra can display the top and bottom screen of the 3DS separately.
This accessory is very niche, with even Asus themselves discontinuing support on their most recent ROG phones, but it is clearly possible for it to receive separate top and bottom screen signals, since it is compatible with the DraStic DS emulator.
I’d be happy to mail in my Twinview unit for testing if appropriate, and I know Asus has submitted an SDK for Twinview functionality on the Unity asset store, if that’s of any help: https://assetstore.unity.com/packages/tools/utilities/rog-phone-sdk-162973
Asus ROG Phone 3 ZS661KS
Qualcomm SM8250 Snapdragon 865 5G+ (7 nm+)
Octa-core (1x3.1 GHz Kryo 585 & 3x2.42 GHz Kryo 585 & 4x1.8 GHz Kryo 585)
Citra Android Beta 14