with modern cars now, most of them have on-board computers and electronic this and electronic that..
never have the 2 cars touching or joined together in any way. the polarity may be the same on the batteries but some cars (and i,m pretty damned sure holden are in this category) reverse the polarity for their on-board electronics..
holding/touching both cars is classed as joining the cars together,.
to stop the risk of melting your on-board computer,i undo the terminals from the good battery so its in no way connected to the car.
if there isnt enough grunt in the good battery,then perhaps the other car has more serious problems.now i ring a breakdown truck. i,m not game to open the bonnet any more.
those computers/electronic bits can set you back major $$$ ,so if in doubt, get a mechanic or someone who knows what they are doing.
fan-baller is right about the explosion risk, they can be instant and very serious....