I didn't want to do the math so I just ran a sim. Let's suppose you want to play a bone wall (5 
quantums needed) and an elite otyugh (5 
quantums needed.) If you draw 5 pillars from a mixed batch of gravity/death pillars you have about 63% chance of having the quantums you need for each after 3 turns. If you draw 5 dual gravity/death pillars you have about 88% chance of having the quantums you need after 3 turns.
Based on your estimates, it looks like you did do the math! Or at least had tons of trials. The explanation is below if anyone is interested -- if not, skip down to the "****".
With mono-pillars, you essentially need to get 2 or 3 heads out of 5 fair coin flips (heads is gravity, tails is death). There are 2^5 = 32 equally likely sequences of flips, including 1 each with 0 and 5 heads, 5 each with 1 and 4 heads, and (5 * 4)/(2 * 1) = 10 each with 2 and 3 heads, so the probability of success is 10 * 2 / 32 = 5/8 or 62.5%.
With dual pillars, you need between 5 and 10 heads out of 15 flips. There are 2^15 = 32,768 possibilities, including 15!/(10! * 5!) = 3,003 each with 5 and 10 heads, 15!/(9! * 6!) = 5,005 each with 6 and 9 heads, and 15!/(8! * 7!) = 6,435 each with 7 and 8 heads, so the probability of success is (3,003 + 5,005 + 6,435) * 2 / 32,768 = 14,443/16,384 or 88.153076171875%.
Another sim: suppose you want to play something costing 7 quantum. What is the chance you will be able to play it after 3 turns, given 5 pillars? With mono-pillars, the chance is 48%; with dual-pillars, the chance is 69%.
The sim wasn't quite as close here. Now you need 2 or fewer out of 5 heads with mono-pillars (chance of success 1/2 -- you just need more tails than heads), or 8 or fewer out of 15 heads with dual pillars (chance of success 1/2 + 6,435/32,768 = 69.6380615234375%).
Also, **** that 7
shows up as the cost of bone wall, when I look in the bazaar. Is the 5
for an upgraded bone wall?
Objection #2 (from Zanz): "Graphics for sixty new cards? Help!"
I would just have the machine generate the dual-pillar graphics, each pillar graphic from each of the two source pillars laid side-by-side in the new dual-pillar graphic. Maybe you could even generate them at runtime, if that helps.
Putting them side-by-side, even compressed into one frame, would look to me like the card was equivalent to 2 pillars, producing one of each quantum per turn. If this idea is implemented, perhaps superimposing the images would be more intuitive.
