Is it possible for different patent agencies to represent pct international segment and national segment respectively?

yes.

the international phase of p>PCT is usually done by an agency; When PCT enters the national stage, different agencies will be adopted in different countries, because there are few (even no) transnational patent agencies. In this way, when you enter a country, you must find an agency in that country to help you apply for a patent, answer the examination opinions and other affairs.