According to the Wikipedia article for the Oklahoma Territory, Theodore Roosevelt chose to merge the requests for statehood of the Indian and Oklahoma Territories.
The [Sequoyah Constitutional Convention] drafted a constitution, drew up a plan of organization for the government, put together a map showing the counties to be established, and elected delegates to go to the United States Congress to petition for statehood. The convention's proposals were presented in a referendum in Indian Territory, in which they were overwhelmingly endorsed.
The delegation received a cool reception in Washington. Eastern politicians, fearing the admission of two more Western states, put pressure on the U.S. President, Theodore Roosevelt. He ruled that the Indian and Oklahoma Territories would be granted statehood only as a combined state.
However, Article 4, Clause 1 of the US Constitution states that it is Congress' duty to admit states into the Union and does not give any responsibilities to the President.
New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
Given this, what gave Theodore Roosevelt the authority to merge two requests for statehood into one?