Dual Stream will use part of the main stream bandwidth.
The main stream is the video payload based on the call setup/negotiated bandwidth less audio payload.
You can configure this on the Enpoint, which part to took from the main stream (for example 50%).
384k call (aprox. 460kbps on L2 network)
64k for AAC-LD audio codec
320k for video payload
if configured to use 50% (160k for main video and 160k for dual stream)
Regards