Why is Brunei separated into two parts?

What do you mean by 'parts'? Are you talking about the social equality or something?well, if it is, then, everyone is treated the same way as others. Atleast that is what I could see. If you are asking about the state having two parts, then, Brunei doesn't have any state. Brunei contains 4 districts. So, I don't tis actually help, but oh well, work with me here, ahahha

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Brunei is geographically in two parts which are separated by the district of Limbang which is part of the state of Sarawak, Malaysia. This was caused by the annexation of Limbang by the Rajah of Sarawak in 1890.