A user from Brazil has posted a request for main/sub title splits to the Zotero forums.
As I note in the thread,
citeproc-js is capable of doing this. While I mentioned in the thread that “the default could be changed,” I won’t be changing the default in the processor source, since the variables “title-main” and “title-sub” that it makes available are outside the CSL specification. The feature would need to be enabled in the processor by the calling client. 
I’m putting this up because the feature would solve a problem for Brazilian users. If eventually adopted in CSL, it should be safe to mandate in the specification and deploy as default behavior in processors—but whether to do so is of course not my call.
The splitting logic is the same as that used for subtitle capitalization in APA and a few other styles, and is illustrated in this test fixture.
 As an implementation-specific detail, the option would be enabled via the sys object used to instantiate the processor, with something like:
sys.main_title_from_short_title = true;
(And yeah, the option is badly named. It dates from the time when main/sub splits required matching content in the shortTitle field, which is no longer the case.)