I don't know if there were tests on this to say otherwise, but basing on the usual mechanics of the game, you actually need more of both to cap at 50%. Fast cast and -song casting aren't added together, instead, their delay should be multiplied. For example, using that 25% for both we would get 75% casting delay. 0.75 x 0.75 = 0.5625 so your delay is actually 56.25% of the base one (or you only reduced your spellcasting by 43.75%). You need a bit more of either fast cast or -song casting to cap it for real.