For the Tenebrium skill, level 1 allows you to use and handle tenebrium weapons. Level 3 allows you to add tenebrium bars to (most) weapons to add that damage type, but only 1 person would need to get to that level, possibly using equipment bonuses. Higher level tenebrium weapons may require up to level 3 (IIRC) Tenebrium to be able to use, but you aren't really missing out if you can't use them.
The Tenebrium ability doesn't boost weapon damage, or weapons directly, so doesn't stack with any weapon abilities.
