If you are seeing 2 separate boils at the surface the disks are definitely too far apart. I like to keep disks on my manifolds almost touching maybe no more than 1" apart or touching so I know the two water columns conjoin ASAP. The deep the diffuser the more water that will end up in the final flow at the surface.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management