When adding a %X attacks deal X damage gem to an item that already has this effect from an enchantment, it does not seem to work atleast if its two different elements. I haven’t tried two of the same type yet.
For example attacks deal cold enchantment with an attacks deal plague damage gem does not seem to work, it doesnt show a damage number (only the cold shows) may still be doing damage/apply status but i dont know.
I hope it’s intended to work togther because then i can apply multiple status to enemies. 
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is this a known issue?
Apparently there is already a bug report to this 
Let me just drop some stuff here:
I enchanted a claymore (base 10) and it got 11% shock dmg. Then I gemmed it with a fire gem (12%) and that apparently overwrote the 11% lightning entirely.
And I enchanted another claymore (again base 10) and it got 15% fire dmg. Then I gemmed it with a fire gem (7%), so the fire would be 22% total and that increased the fire dmg from 4-5 (before) to 6-7 (after).
So this works.
→ If you have elemental damage on your weapon, only a gem of the same element will keep the enchantment alive!
Link to related topic: What weapon gem to use
I hope they allow multiple damage types to work together soon. Nothing more fun than having a fire/ice weapon 