The idea here is each exercising their own authorities, right? Parliament has set an upper limit, for example, for 30 grams in public. A province, for its own purposes, could then set it down to 15 grams.
Where we have limits to this kind of range has to do with the double aspect doctrine. Where compliance with one act requires non-compliance with the other—a province brings it down to zero—or where the purpose of a federal act would be frustrated by the provision of a provincial one—again, a province bringing it down to zero—this could give rise to paramountcy. In other words, a court would examine whether there is a conflict, or whether the purposes of the federal act are frustrated, and could find the provincial law inoperable to the extent of that frustration.
When we're looking at a situation where Parliament is wanting to allow access to a lawful supply by adults and a province enacts legislation that interferes with that ability to the point where it is zero, then a court, if challenged, could look at the situation and see to what extent Parliament's law has been frustrated.