After they get PR, there is no PA anymore. Both of their PR's are equal and the same.
Only the spouse that has been in Canada for 3 years can apply for citizenship, doesn't need to be continuous but 3 years out of the previous 4 before they apply. If they have minor children, the spouse applying can add them to the application but the children do not need to fill the 3 year requirement. The spouse who is staying away will not gain the right to apply for citizenship and also has to fill the residency requirements to keep their PR which is living in Canada for at least 2 years out of each 5. If the Canadian citizen spouse joins the PR spouse after getting citizenship, as long as the PR spouse had not already lost their PR, they will not lose their PR because of residency requirements as long as they are living with the Canadian citizen spouse.