Lets look at similarities:
Haste Grants an extra attack due to an increase in your speed & action of movement (despite weapon)
Blessing of Fervor can grant an Extra attack do to an increase of Speed & Action (similar to Haste, also despite weapon)
Speed property allows you an extra attack with a weapon do to the weapon being faster & increasing your action.
By the logic you're using 2 speed weapons are 2 separate sources that are not "similar". By that Logic does that mean Haste & Fervor should stack? they are more likely considered separate sources than 2 weapons of Speed.
If the Answer to this question is yes...congratulations you just made a house rule that ignores RAW enjoy it, have fun, More power to you.
Each of these effects is similar in that it manipulates speed in a localized area to give you an extra attack, this seems to be a logical, balanced way of interpreting the rules as they stand.