Bilingual Subtitle Merger

Language learners and translators constantly pair two subtitle tracks: Chinese on top of English, Spanish over Japanese, any combination. This merger takes two single-language subtitle texts (SRT or WebVTT, auto-detected) and produces one bilingual track where each cue stacks the primary language above the secondary. The two tracks rarely time identically, so alignment runs first: by default each primary cue is paired with the secondary cue that maximises time overlap (intersection-over-union), with a nearest-start fallback inside a tolerance window — the same strategy dual-language mergers in the wild use. Tracks that share a source (a file and its translation) can instead pair 1:1 by cue index. You choose which track's timings the merged cues carry; cues that exist in only one track are kept as single-language cues so no dialogue is lost. Input and output stay plain text — nothing is uploaded.