| jhansonxi |
Magical Knack doesn't work for Alchemists because they aren't spellcasters.
The Underground Chemist rogue archetype gets access to alchemists discoveries at 10th level.
These two classes have an inherent synergy and complementary features (like skills), making the combination appealing to PCs with both high dexterity and intelligence (goblins, tieflings). It also makes for a good story - the amateur chemist decides to become a professional alchemist.
Allowing the former to stack with the latter when determining DC, duration, and caster levels of effects and skill checks would not be overpowered and help negate some of the limitations and oddities of alchemists as non-caster "casters".