The only simple answer your question has is that the largest individual during each encounter wins. Even this though is not guaranteed, as a multitude of factors (age, size, health, venom resistance, etc.) can come into play altering the outcome of your question. If an adult mulga comes across a young blackhead then the mulga wins and vice versa. This will also hold true for king cobras in Asia. Even though they may be the largest and 'strongest' as adults, they don't start that way, and I'd dare say quiet a large number of hatchlings/young individuals would fall prey to other ophiopagous species. Simply the biggest of the pair during the encounter will be the winner.
I also have to ask what is your fascination with this subject? The majority of if not all of your posts are either asking about this questions or related to it in someway. So I am curious as to why this is such a crucial question for you to have answered?
Cheers, Cameron