Yeah, I think the term dual fuel means using both fuels simultaneously in whatever the optimum ratio. But, we use it in common terms for Bifuel(one fuel at a time).
Fellas in Perth apparently don't like the way we(VIC) use those terms.
So dual fuel in colloquial terms in Victoria means Bifuel in Perth, and perhaps in NSW as well?!
Back in late 90's MET buses in Melbourne had a all their buses converted to Diesel+LPG injection system(purely for economy I reckon).