Yes, you are very late to the party.
They cancel each other out completely, leaving the ordinary potency/tenacity check there....the same way it worked before, except now you can actually see both icons.
Tenacity Up adds 999900% tenacity, which effectively acts like debuff immunity, except against unresistable effects, Tenacity Down being one.
Tenacity Down adds -999900% tenacity...the difference here is that there is a 15% base chance to resist. So debuffs still only have an 85% chance to land when they are attempted. I will make no secret of the fact that I think this mechanic needs to either go entirely, or be changed to 2% like dodge.
