It’s a descriptive error. Even though it doesn’t show 4.8 sec(which is 20%+20% = 40% -> 0.4 X 8 = 4.8 sec), stacking Greatsword Training & Illusion Celerity do indeed give your MB a 4.8 sec CD. It’s just that the CD number “in the description” remains at 6.5 seconds despite the fact that the proper CD number should show 4.8 sec.
I can confirm that for stacking the traits MB and GT, but as for other CD traits combination they are held in seperate cases in terms of whether they do or don’t stack – and I believe that’s how it works.
CD reduction from GT only stacks for skills already affected by Illusionists Celerity. I can confirm the descriptive error on Mirror Blade and Phantasmal Berserker, however on other skills the cooldown is still the same.