The speeder bikes have impact 1 on the card and 3 attack dice. When you roll, you get 3 dice per bike, but do you also impact 1 per bike or is it impact one for the entire 6 dice pool?

the weapon has impact one. you have two bikes. therefore you have two weapons. so Impact 2.

Take a look at the new FaQ thread which explains this (although it does it for grenades)

A full unit of bikes has Impact 2. After one dies, it drops to Impact 1.

Anything listed on the bottom of the unit card, where the weapons are, is a weapon keyword. Anything listed on a card granting you the use of a weapon is a weapon keyword - you can also see what is a weapon keyword by looking at the RRG under "weapon keywords." Weapon keywords stack for the number of times that weapon is in your attack pool.