To split one subtitle into two there are two methods for which you can assign different shortcuts:
Split title at cursor position
Will split the title at cursor position for the text, but the timings of the two new subtitles will retain the in cue and out cue of the split subtitle (unless it breaks a validation rule).
The two new subtitles will be half the length of time, regardless of where the cursor was placed. You will then need to adjust the timings accordingly.
Split title and duplicate text
Will split the title into two, at the current time position. The original text is duplicated in both titles for you to edit.