Regardless of the country, and whether you are an electrician or not, let's stop and think about this for a minute:
Say you have a two-gang metal box in a kitchen housing a switch for the light over the sink (on the lighting circuit) and a receptacle on one of the small appliance circuits. Using Romex (Loomex), how in the world could you possibly keep the grounds separated? Even if you didn't bond them together, there's no chance that those bare conductors aren't going to meet, either by touching the box directly or via the grounding connections to the device yokes. The separate grounds are going to meet at some point. I'd tend to think that an intentional bonding of EGCs would be better than those that may occur through unintentional contact.
I think that with this being said, the answer is obvious.
I think that any code that requires that such circuits be kept separate would be impossible to enforce unless separate device boxes were mandated for separate circuits. It would be even more difficult to comply with such a requirement. I think that pretty much sums up the answer to the original question.
"But the guy at Home Depot said it would work."