原文：
Select a secondary subtitle stream. This is similar to --sid.
If a secondary subtitle is selected, it will be rendered as toptitle
(i.e. on the top of the screen) alongside the normal subtitle,
and provides a way to render two subtitles at once.
There are some caveats associated with this feature. For example,
bitmap subtitles will always be rendered in their usual position,
so selecting a bitmap subtitle as secondary subtitle will result
in overlapping subtitles. Secondary subtitles are never shown on the terminal
if video is disabled.
Note
Styling and interpretation of any formatting tags is disabled
for the secondary subtitle. Internally, the same mechanism as --no-sub-ass is used
to strip the styling.
Note
If the main subtitle stream contains formatting tags which display the subtitle
at the top of the screen, it will overlap with the secondary subtitle.
To prevent this, you could use --no-sub-ass to disable styling in the main subtitle stream.