My question refers to the practice of tosafos Shabbos, or extending the duration of Shabbos. In Yoma 81b this is called a mitzvah, and we learn one is required to do so at both the beginning and the end of Shabbos--but one only needs to do it by very short times, which are probably already reflected in the published z'manim. However, I have heard in a few places that very pious people may extend the Shabbos even longer, and that this is considered meritorious.
My question is this: if one is deliberately extending one's Shabbos, and others are not, is it halachically acceptable to derive benefit from melachos they are doing during your extension? On one hand, they are not doing the melachas in violation of Shabbos--but on the other hand, shouldn't your observance stay exactly the same throughout your Shabbos?
This probably connects to the question of whether you can derive benefit from the melacha of a Jew in another part of the world for whom it is not Shabbos...