The constituencies - Pakke-Kessang in East Kameng district and Likabali in Lower Siang district had remained vacant for the past several months.
The notification for the by-poll would be issued on November 27 and the last date for filing of nominations has been fixed on December 4, while scrutiny of papers would be done on December 5.
Last date of withdrawals of paper would be on December 7 next and counting of votes would take place on December 24.
With the announcement of the by-election, the election model code of conduct has come into force in both the districts.
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The constituency has 26 polling stations.
The Likabali constituency with 39 polling stations has total voters of 10608 including 5331 female voters.
The Pakke-Kessang seat fell vacant after the Gauhati High Court declared null and void the March 2014 election of former deputy chief minister Kameng Dolo following an election petition filed against him by BJP nominee and former minister Atum Welly.
The Likabali seat fell vacant after Health and Family Welfare Minister Jomde Kena died on September 4 this year.