I assume they don't, because durations don't usually stack in ESO. Usually whichever duration is longest of the two being compared is the one that applies, as far as I know.
If i remember correctly game checks both sources of Major Force and gives you the longest one so they do not stack but still set works and results in more Major Force time.
Saxleel is generally better on a necro tank or healer with colossus than with warhorn.
Yep this. Spending 250 ult for a Warhorn normally gives 10s of Major Force, while a Saxlheel gives 16.7s of Major Force (not 10 + 16.7).
This means the set only adds an additional 6.7s of Major Force, and Warhorn can be used approximately once per minute, so and increase of approximately 11% uptime. While Major Force is active it increases group DPS by about 6%, so 11% uptime only results in ~0.7% group DPS. IMO this is not enough to justify using it, since there are numerous support sets with more impact.
Now if the Sax tank was not planning on using Warhorn that changes everything. Now all of the Major Force duration is from the set and it ends up being around 1.7% group DPS minimum (higher if they are doing something like Necrotic Potency between Colossus casts on Necro or Replenishing Barrier spam). This still falls on the low end of support sets, behind Yolnahkriin, but ahead of Torug’s Pact, roughly equal to Worm.
Also be sure that the other tanks/healers are keeping the 10% resource buff from Horn active at all times, just use the Sax tank to fill gaps in Major Force.