No, you only get a total of +1 regardless of how many ships have spotted the target. What you do get is redundancy. If one ship has spotted the target and is then destroyed, you don't get the bonus any more - at least, not until another ship spots it. If two ships have spotted the target and one is destroyed, you still get the +1 bonus because the second ship is providing it. Note that "successfully attacked" does not require an actual hit. So long as one of your ships has successfully rolled to beat the enemy's stealth, that counts as a successful attack for the purposes of bonuses to other ships.
Likewise, you only get +1 bonus regardless of how many ships have successfully rolled their Scout trait. But the Scout bonus does stack with the bonus for a ship having attacked the target. So if at least one of your scouts has used its trait then the first ship to attack the target gets +1, and if it successfully rolls against the target's stealth then the next ship to attack that target gets +2.