Weak Memory

The main problem with making two pills at once, from what Alex had come to realize after a bit of testing, was Heat. If you wanted to make two pills at once, and truly make it at once and not one after another or alternatively, then you needed to make it so that your cauldron had two different temperatures at once.

The upper half of the cauldron, where one pill would be made, would have to have a different temperature from the other half of the cauldron. That required one not just to have a Dao of Heat, but also a Dao of Heat Conduction to stop the heat from flowing from one section to the other. 

On top of that, the different temperatures in the same cauldron would cause the cauldron to have a durability problem.

Alex worried Memory would start showing such problems as well. Unlike Midnight, which he had kept with himself for the past 2 decades, Memory had been lost to him for a majority of that time, and had started making pills with it half a decade ago.