Yes, you need to learn even the shield mastery. The explanation (as far as I recall) is: weapon masteries "train" the right hand, shield mastery is needed to
train" you left hand. And for dual wield a hero needs both hand, so that's why you need all these skills to be at lvl 7.
And yes, even one skill point at Dual wield is enough to unlock the skill. However, a shadow stalker can use ONLY two daggers simultaneously. Still, their damage, block, critical, attribute and % bonusses stack up (so you will have extra 40% Agi and Dex).
A fury warrior with the Dual wield skill can equip one-handed swords.