I wouldn't be surprised if in fact it was double early on where the air is markedly thinner.
FFS nojwod he was only at 7000' not FL270
The worst performing (on one engine) piston twin I ever flew, the Twin Comanche, would **** it in from 7000'/25 odd nm if the only issue was 1 engine shutdown.
He wasn't much above 'training' weight with two up and Brisbane fuel. Name me a piston twin in commercial usage in 2010 that won't fly level below 5000' on a winters day at training weights?
There isn't one. Most piston twins could fly around at 7000' on one engine all day lightly loaded in winter - even if that was not feasible the resultant drift down rate with one set at MCP and one feathered would be in the order of several hundred feet/minute, tops, until reaching an altitude it could maintain - unless you ran a tank dry in the process.
I dare say the ATSB will start with fuel - did he take off with the selectors on partially filled aux tanks and when the first stopped was he so preoccupied with getting back to YSBK he never checked/changed tanks and subsequently ran the second dry a few minutes later?
That
sort of scenario has been repeated down through the decades so many times you really wonder at peoples inability to learn from history.
The chances of BOTH engines developing a mechanical fault within minutes of one another is vanishingly small. Either he was faced with another 'Spencer Gulf' type scenario (I would be gobsmacked if it turns out to be so) or he cocked something up.
It will be interesting to read the report when it comes out but I suspect it will hold only a reminder of basic good airmanship rather than a re run of Spencer Gulf. Even that young man managed better than 25 nm before succumbing to fate.