The extra attack granted by Speed special property does not stack with Haste:
When making a full-attack action, the wielder of a speed weapon may make one extra attack with it. The attack uses the wielder’s full base attack bonus, plus any modifiers appropriate to the situation. (This benefit is not cumulative with similar effects, such as a haste spell.)
Which is also repeated on Haste spell effect:
This effect is not cumulative with similar effects, such as that provided by a speed weapon
That said, what is called haste effects all work similarly, it grants you an extra attack using your highest attack bonus. Iterative attacks always go from the highest to the lowest, and off-hand attacks from two-weapon fighting must always go from the highest to lowest aswell, following the main-hand attacks.
Examples (i will bold attacks granted by haste effects):
- You have +5 BAB (still one iterative attack), and an off-hand weapon, so your attack bonuses are: +3/+3
Same setup, but now you got hasted. First attack must be main hand, second and third you can pick either a main hand or offhand attack: +4/+4/+4
Now you have +6 (two iterative attacks), and an off-hand weapon: +4/+4/-1
Same setup, but now hasted: +5/+5/+5/+0
Let's add Improved TWF (second off-hand attack): +4/+4/-1/-1
- Same setup, now hasted: +5/+5/+5/+0/+0
Regardless of which weapon has Speed, the attack must use your highest attack bonus, so it will be used after your main weapon, then followed by off-hand and remaining iteratives.
Haste allows you to use any weapon on this extra attack, while Speed restricts your attack to the weapon that has the ability, that is the only difference between them. So if both weapons have Speed, you can pick each one you will use each round, similar to Haste.
This can be seen on this post by James Jacobs (Paizo's Creative Diretor) aswell.
When an ability allows you to make an extra attack that stacks with Haste, this is called out. Like on the Name-Keeper Shaman archetype:
Swords: The name-keeper gains proficiency in all martial weapons and one exotic weapon of her choice. At 12th level, whenever she makes a full attack, she gains one additional attack at her highest base attack bonus; this ability stacks with the extra attack from haste, but it doesn’t stack with other abilities that grant extra attacks.
Note that this ability stacks with Haste, but not effects similar to haste, such as Speed.