If you apply multiple elemental enchantments the previous gets replaced.
Same is true for Tenebirum. You can only have either elemental damage or Tenebrium damage if you coat a weapon with the exception of magical weapons with already existing elemental damage. In that case you could add Tenebrium damage to the elemental damage. On the other side, you can't enhance (real, not coated) Tenebrium weapons with elemental damage.