You do less then base damage with grit + darside
Basedamage - Grit = Gritdamage + Darkside = combined Damage:
100 - 20% = 80 +20% = 96
Basedamage + Darkside = Darksidedamage - Grit = combined Damage:
100 + 20% = 120 - 20% = 96

As you see you do about 4% less damage in grit + darkside as you would do without both.