Which two provinces joined Canada in 1905?

To "join" implies that two entities existed separately, and then became one.

While Alberta and Saskatchewan became provinces on September 1, 1905, it was because they were created out of the Northwest Territories by the Government of Canada. These provinces did not exist as separate entities before September 1, 1905, and therefore could not "join" Canada.

It is better to say that they "entered Confederation" on September 1, 1905.