Different machinery complicated battleship division planning

Why did the United States build battleships in pairs but not always operate each class as a two-ship unit?

Sister ships could use turbines and reciprocating engines, making them less natural partners for a permanent operating division.

Several early American battleship pairs were machinery experiments as well as ship classes. One sister might use steam turbines while the other retained vertical triple-expansion engines. The ships could operate together, though their different machinery made them less natural partners for a standing two-ship division. Procurement experiments could therefore influence fleet organization long after construction ended.