You can rear them together. In fact, there was a program (clandestine) that reared summers and coho in the same raceways. Worked very well.

The problem, I believe, when they started to rear steelhead at surface water stations (salmon) is thatchy water is colder, lots colder. Prior to merger, steelhead were spawned, incubated, and often reared on ground water which is warmer. They evolved, genetically, to function in warmer water. Put them into the much colder surface water and the enzyme systems don't work so they don't perform well. Taken long enough, they can evolve back to the colder water.