Is a researcher who is affiliated with one university in country A, within the scope of a joint doctorate and wants to apply for a Postdoctoral Fellowship with another university in country A eligible?

It is the physical presence (residence or main activity) in a given country that is taken into account when considering the mobility rule. If they have not resided or carried out their main activity in country A for more than 12 months in the last 36 months, they are eligible.