I was trying to fire 2 unfired crucibles by placing them in the same slot in a small vessel, but I got only 1 crucible.
I guess the key is that the crucible is unstackable while the unfired crucible isn't.

TFC version: 3.2.7
I am playing tfc by ATMG2, I am not sure whether it does matter.

This is works as intended; although we could make the unfired crucible not stackable to prevent the confusion in future. But in general, any heat recipe from a stackable -> unstackable item will have this issue.