I rip my Blu-Rays and DVDs, convert to M4V with Handbrake, and create SRT subtitles through OCR, which I then mux into the M4V with Subler (which translates them to "Text 3GPP").
If I ever place more than one subtitle track (foreign-language-only tracks vs. full SDH tracks, or even commentary subtitles, for example), I can access all subtitle tracks on my Mac with iTunes, and also on my Apple TV.
On my iPhone, though, I can only access the first subtitle track. If there are multiple audio tracks, those are available to me, but on a video that I can confirm has multiple subtitle tracks, the Videos app only shows me the first one. The others aren't even options.
Is there some trick to encoding or muxing? Or does the Videos app on iOS only support a single subtitle track?