The by-poll to Tura was necessitated due to the demise of former Lok Sabha Speaker Purno A Sangma on March 4.
The Assembly by-poll is required as sitting Congress MLA Gabriel Denwang Wangsu has resigned from the post.
The notification for both the by-elections would be issued on April 22 and last date for filing nominations has been fixed on April 29, the EC said in statement.
Scrutiny of papers would be held on April 30 while the last date of withdrawal of papers is May 2.
With the announcement of elections, the model code of conduct has come into force, the release added.