Originally Posted by FAQ 3.0
The description for the speed weapon property says it is not cumulative with the haste spell, but exactly how this applies in the game seems unclear, especially when two weapons of speed are involved. Can you use two speed weapons at the same time? What, exactly, is meant when the rules say haste and speed don’t stack? Does it mean you can use the speed property during the normal portion of your turn, but not during the “extra” partial action you get from haste?
You can use two weapons of speed at a time all right, but you still only get one extra attack, from one weapon or the other, not both. If you are under the effects of a haste spell, you can use the extra attack action from one weapon of speed, or the extra partial action from the haste spell, but not both.
3E的FAQ. 虽然不知道有多少参考性.