You'll find more good DPS advice/guides/activity here: http://www.tankspot.com/forumdisplay...ond-Discussion
This idea is new to me. You're saying you have found advice to let one disease fall off before refreshing but not the other? In general DoT maximizing is a little tricky. If you time it poorly you can essentially lose a tick, and for certain specs that can be an issue worthy of attention (really Afflic Locks are the ones who care most about this). Unholy, of all the DK specs would care about it most, but it is still not *that* big a deal that you need to get too fancy with it. Reapplying a disease right after it falls off makes sure you don't waste ticks, but you can accomplish the same (albeit less efficiently) if you just time it on the 3 sec interval before hand.
So far as I can figure, the only reason you would only let one fall off at a time, like you're describing, is just an act of staggering your diseases so that they don't run out at the same time. All of this is tempered by the value of having both diseases up for the sake of the rest of your abilities, the buffs you mentioned, and the per-disease damage buffing of your other abilities.
Maybe it can be cleared up if you can explain better just how you are doing it.