This is a question that often comes up, but despite the initial attraction of a twin engine installation, they tend to be quite rare in practice.
The exceptions are lifeboats and fast rescue craft who absolutely must keep going at all costs, and large RIBs in the USA where petrol is cheap enough to make a pair of 250 hp outboards viable!
For serious cruising boats a single diesel engine is becoming the preferred choice. Diesel is often more widely available at the quayside -- wherever you have fishing boats you'll find diesel -- and lugging hundreds of litres of petrol in jerry cans is no fun!
Diesels tend to me more fuel efficient than outboards, although with the new technology the gap is barrowing. In countries where marine diesel has a lower rate of taxation like the UK the cost benefits are huge if you do a lot of boating (it's also psychologically comforting to fill up cheaply each time, even though you may have spent substantially more buying the boat in the first place).
With a twin installation there is twice as much to maintain, as well as twice as much to go wrong! A twin installation is also less efficient than a single of the equivalent combined horsepower (eg 2 x 100 hp is roughly equivalent to a single 150 hp in performance terms).
A well maintained diesel engine is a very reliable piece of kit these days, especially when fitted with a substantial fuel filter and water separator to cut down on the chances of fuel problems.
You could always carry a small outboard as a reserve engine. This has it's own problems of a separate and limited fuel supply, plus secure storage, but is quite a common solution. I suspect, however, that quite a number of these engines get very little maintenance and may not be up to the job if things got serious enough to really need them.
A few of the bigger RIBs have been fitted with twin diesels, but this is out of the scope of most RIB cruisers!
A final option is a diesel outboard as a reserve. This avoids the problem of carrying a separate petrol supply, but they are very heavy and not widely available. This is the option that Alan Priddy has gone for in "Spirit" his round-the-world RIB.
What are the conditions like off Queensland? Will you be on your own, or with another boat?