Hi aegus. They're most likely using an external controller which triggers simultaneous playback start via GPIO or RS-232 or ethernet. Either a cheap DIY variant or maybe a pricey NA10. That's not _synchronized_ playback, though, more like throwing 2 balls at the same time.
Unless there is absolutely no way for them to simply add the subtitles to the package this is completely nuts, too, of course
Looks, readability, color temperature, distortion ... none of these will top "barely acceptable".
What's the country and language in question here?