To add, if you set both torpedoes to slow then you get more time to get away, and you'll get a bit more visual separation in the scope. If you set them to fast then they (targets) have less chance to react but they might overlap visually in the scope when you are about to shoot.
I assume you set the speed of the target correct, and you don't so something silly as firing gas proppelled Type 1 torpedos in broad daylight.
