Honestly, as someone who is doing an eng degree (1 year to go ) at Melbourne.
I did BSc and am now doing M.E.
In retrospect, I would've picked Monash, strictly because I'd like to save that year but then I don't know. I'm trying to get a job in oil engineering/aerospace engineering in Qatar at the moment and they keep telling me to specify if I have a Masters or PhD so who knows, maybe my extra year will pay off.
Honestly, I don't see the point of doing a Comm/Eng double unless you plan on going into commerce. Engineering degrees are very highly regarded in the commerce sector, the converse isn't true, no-one really gives a shit about your commerce degree in the engineering sector (from what some BCom/B.E from the old model) tend to tell me. Commerce degrees are genuinely regarded as easier to complete than engineering ones in the work-force.
Don't do both for the sake of it, if commerce and engineering both interest you, take a double. Otherwise pick one.
If you live closer to Monash or Melbourne, pick that one. Otherwise, they're both 90% identical.