What you want IINA to do:
Provide a way to separately control the delay for the primary and secondary subtitles. This should include a way to keep the delays in sync to avoid forcing users to change two sliders when both subtitles need the same delay applied.

This feature request comes from a Telegram user in the IINA group chat.

What IINA does currently:
A single slider on the subtitles panel controls the delay for both the primary and secondary subtitles.

Why you think this should be added:
This ability has been requested in mpv issue mpv-player/mpv#2629. There is a pending implementation in mpv PR mpv-player/mpv#11689.

This IINA issue tracks the need to update IINA once mpv has added this feature.