Extand stay and re-entry question

[S.Z]

Hi there,
My parents are going to apply for extand stay with regular visitor visa. I wonder if they get extra 2 years authorized stay, but don't stay in Canada for 2 years.
. Assuming they stay in Canada for 6 months, then go back home country for 2 months. When they come back to Canada, will the 2 years authorized stay still be effective or automaticity replace by the regular 6 months?

 

[Buletruck]

Each entry is assessed on arrival. Chances of a 2 year extension, In my opinion are low on a regular visa. And re-entry doesn't guarantee they will get 6 months (or even be admitted) if CBSA feels they are not genuine visitors (I.e. Spending more time in Canada that at home).