Dar al-Harb is defined as every place where the laws of the Kufr are imposed (al-Insaf fi Ma‘rifat ar-Rajih min al-Khilaf). Hence it is does not make sense to say that an area is Dar al-Harb for some Muslims and not others. Being Dar al-Harb is the default status of every place that is not under Islamic rule.
The disbelieving residents of the Dar al-Harb can be either Harbi or Mu‘aahid. The default status is Harbi, i.e. every disbeliever among them is a Harbi unless he has a treaty with the Muslims. Further, even if he has a treaty he would return to being a Harbi when he breaks this treaty or when the duration of the treaty expires. It should be kept in mind that a treaty of this type is not permissible unless there is a need or an interest served by it, and it must have a defined duration so that Jihad can resume and will not be abandoned. Also treaties need to fulfill certain conditions to be valid and are broken by violations against the Muslims by the enemy.
It is possible for some Muslims have a treaty with a faction from the Dar al-Harb while other Muslims do not. Some madhabs hold that if there is a dire necessity the Caliph may agree to a treaty with the enemy whose protection does not extend to all Muslims. For example the Prophet ﷺ made a temporary ceasefire with the Quraish at Hudaibiya. According to the original terms of the treaty any male Muslims who were still left in Makkah had to be returned to the Quraish if they were to escape to Medinah. As a result some of the Muslims who managed to flee from Makkah after the treaty had to be returned, but they later formed a separate group and fought against the Quraish. Hence this was a situation where one group of Muslims were at war with a people while at the same time the rest of the Muslims had a treaty with them.
وينفلت منهم أبو جندل بن سهيل، فلحق بأبي بصير، فجعل لا يخرج من قريش رجل قد أسلم إلا لحق بأبي بصير، حتى اجتمعت منهم عصابة، فوالله ما يسمعون بعير خرجت لقريش إلى الشأم إلا اعترضوا لها، فقتلوهم، وأخذوا أموالهم
Abu Jandal bin Suhail got himself released from them (i.e. infidels) and joined Abu Basir. So, whenever a man from Quraish embraced Islam he would follow Abu Basir till they formed a strong group. By Allah, whenever they heard about a caravan of Quraish heading towards Sham, they stopped it and attacked and killed them (i.e. infidels) and took their properties.
— Bukhari
