We have minor and major buffs for each and every effect, like increase weapon damage in this case. The only exception is Empower, which has only one type (+20% dmg for next attack). The same type of buffs do not stack, so you can't have neither the benefits of a minor + major buff at the same time, nor the same buff twice: minor+minor or major+major.

The only buff that stacks is actually the mentioned buff above, Empower, so you can have a buff + Empower on a single attack at the same time. Please also note that DoT abilities and channeled abilities only proc the Empower buff upon the initial hit.