Direct answer
Merge subtitle tracks or create bilingual cues
The subtitle merger supports two different jobs. Sequential mode places the second track after the first with a configurable gap, which is useful for joining parts of one programme. Bilingual mode pairs cues by their order and places both texts inside the same cue. Bilingual pairing assumes the two tracks already correspond cue by cue; if their segmentation differs, review and align them before using the merged file.