Response by Bhai Preetam Singh
Dasam Duar is like a remote access to Sachkhand while being in this human body. Since Sachkhand is a different realm then we are in right now (dharamkhand), it can only be experienced through the Dasam Duar. This maybe the reason that Bhai Sahib has referred to them as the same thing. If someone enters their Dasam Duar then they are in Sachkhand, so saying that Dasam Duar and Sachkhand are the same thing is totally understandable.
Bhai Sahib has written about Dasam Duar very extensively within the book Anhad Shabad Dasam Duar, so I would suggest that as a must read for any serious seeker.
The above is just my meagre understanding of what Gurmukhs have written before, maybe Bhai Kulbir Singh jee can shed more light on this topic.
Response by Bhai Kulbir Singh
Sachkhand is the topmost Khand that is home to Vaheguru jee and Mukat Gurmukhs. One can go to Sachkhand after leaving this body. One can get Anand of Sachkhand only when one leaves this body and goes to Sachkhand but Great Dayavaan Vaheguru made such great exception for Gurmukhs that they can have all Anand of Sachkhand even while residing in this world and in this body, through the secret mystical tenth door i.e. Dasam Duaar. Dasam Duaar is such a secret doorway, within this body, from where one can access Sachkhand and its Anand.
Dasam Duaar and Sachkhand are not the same thing. Dasam Duaar is the secret doorway within this body, from where one can access Sachkhand but not reside in Sachkhand or go to Sachkhand in person.