If an elite army unit is equiped with 2 heavy venom cannons, can the combat action Area Effect (2) on each card be activated to give area effect (4)?

Thanks in advance

Yes. Each copy of the card has its own limit. You could also give the unit Area Effect (2) and Armorbane (since Armorbane works while the unit is attacking, it works with Area Effect).

Limits and Maximums
“Limit once per X” is a limit that appears on cards that remain in play through the resolution of their effect. Each copy of an ability with such a limit may be used once per each period of X.

Reading this post, two question come to my mind:

1.- I get that two copies of this card attached to a single elit unit will grant +4 attaks, but definitly I'll need to trigger both Combat Actions in order to get Area Effect (4), right?

2.- AS this is a limited action (once per phase), if I trigger the first copy of HVC does it restric the use of my second one?

1.- I get that two copies of this card attached to a single elit unit will grant +4 attaks, but definitly I'll need to trigger both Combat Actions in order to get Area Effect (4), right?

Yes. Of course. The Area Effect is not active until the ability is triggered, and triggering the ability of 1 attachment only grants Area Effect (2). Trigger one, and that's all you get. Trigger both and you get two separate instances of Area Effect (2), which combine to Area Effect (4) as per the rules.

2.- AS this is a limited action (once per phase), if I trigger the first copy of HVC does it restric the use of my second one?

No. That's what Khudzlin was getting at when he posted:

Limits and Maximums

“Limit once per X” is a limit that appears on cards that remain in play through the resolution of their effect. Each copy of an ability with such a limit may be used once per each period of X.