What it means is that the firing time of a dualgunner is equal to 1.2 times the average of his weapons' firing times, or .6 of their combined time.

Thus, a CL3 DG char with unmodded pistols has, (I think) a firing time of .72. The third level of SoG would bring it to .48. I've seen that number before, so I'm pretty certain I'm doing the math right.

If I have the order these things are counted correct, the same character with S1 in each starts at .648, and comes down to .432.

I could be wrong about where exactly the mods fit into this equation, but I think the way it works is you figure the time of each weapon independently, starting with mods, working your way through SoG and Fin for each one, then you plug the two numbers you get into the DG formula.