The 2392 was originally introduced because of the high failure rate of the 2391. It covers virtually all the aspects of the 2391 but at a level that's suitable for less experienced electricians. It is also limited to single phase, hence the domestic comment that is usually attached to it.
The external exam is multi choice, 50 questions and the practicals are assessed as strictly as the 2391 but it is not as difficult and only single phase.
It's probably best looked at as a progression award rather than a full one. The 2391 is designed for those already experienced with inspection and test and looks deeper into the subject, it's assumed that you are more than comfortable with testing already. The 2392 assumes that you have some experience, but want to consolidate before looking at the 2391.
Incidentally the low pass rate of the 2391 is because the course is targeted at experienced testers but a lot of companies don't vet students before they join the course.