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. Whether to use it is not my call.
The option can be enabled in the sys object used to instantiate the processor:
sys.main_title_from_short_title = true;
FB