You run into a problem when you get all death cards and all dark pillars, or all dark cards and all death pillars.
1/6 of each card is a dark pillar. 1/6 of each card is a death pillar. Those aren't good odds to me.
As to two of one and one of the other, even if that's true, averages rarely play out. You're probably just as likely, if not more, to end up with 0 of one type.
All in all, duo decks generally either need one element to be powered solely by mark, or more pillars overall than a mono using the same total quantum.
To the quantum cost, parasites also have 1 death/turn for the ability, and paying for that (even just with 2-3 parasites out, which is really the minimum for them to be useful) will likely take away your ability to play other death cards.