>As far as I know, CO$_2$ and O$_2$ both are almost perfect ideal gas at atmospheric conditions, so the volume they occupy shouldn't affect each other. 

Room temperature CO$_2$ and O$_2$ can often be treated as ideal gases, but this is not one of those times.  The issue is collisions.  The mean free path of an air molecule (e.g. O$_2$) at STP is about 60 nm - an *incredibly* small distance. If you divided a container half using a partition and filled one side with O$_2$ and the other side with CO$_2$, it would take an *extremely* long time ([apparently on the order of 10 minutes](https://physics.stackexchange.com/questions/266836/how-quickly-do-gases-mix)) for the gases to mix completely.

As a result, your CO$_2$ will tend to move as a fairly cohesive mass of fluid on the scale of seconds, which is the time scale you're studying when you dump it out onto a candle. This is also why we talk about [air parcels](https://glossary.ametsoc.org/wiki/Air_parcel) and cold fronts in meteorology - distinct "packets" of air undergoing bulk motion tend to move around each other rather than mix together, because the time required for the latter is far longer than the time required for the former.