Why should i become Stunned when i cross fire with one of my mates against the same target?

Why shouldn't the card read :

When the Attacker 1 declares an attack during his activation u can play this card to make Attacker 2 with LoS to the Target to ADD his ATTACK POOL to yours (against the normal Defense Pool of the defender)

This 2nd Attacker cannot attack again this round.

No Stun condition would apply.

To me one of the MANY mistakes FFG should correct in 2nd Ed.

This is not a rules question.

To your first question: adding 2 dice to a focused Captain Terro attack with a damage power token would make a one-shot defeat attack and you would complain it is overpowered.

To the cost of Stun condition - it is only a loss of one action, and you can choose a Trooper which has already activated and/or will be soon defeated by your opponent.