Tools to merge SRT subtitles
SRTMerger – Input: SRT, Output: SRT, Different colors, displays both subtitles at the bottom, on top of each other.
2srt2ass – Input: SRT, Output: ASS. One subtitle on the top of the screen, one subtitles at the bottom (possible because the output format is ASS).
DualSub – Input: SRT, Output: SRT; displays them at the bottom, next to each other. This is essentially a hack, so it requires you to specify the font with which your player will display the subtitles in advance – you are essentially creating subtitles for a particular player, and it can be a bit fiddly. On the plus side, this one has amazing options for keeping subtitles on the screen for longer, when possible, as well as smoothing over small timestamp differences.