We are not running the two subs off of one channel on the inuke amps. So we aren't running the two subs in parallel or series with each other, they are each being driven by their own channel. So when we talk about the d2 or d4 drivers, the d2 can be wired to be as 1 ohm or 4 ohm load, and the d4 can be wired up as either a 2 ohm or 4 ohm. So for the op, in order to deliver full power to his HT18's from the inuke3000dsp, he wants d4's wired in parallel for a 2 ohm load.
If someone wanted to run two separate subs off of one channel, or bridged off of one amp, that would be a different story.