The way I understand it:
2 or more players declare they are making a teamwork attack before they make their attacks.
They then make standard attacks and cast their spells as normal, rolling their 2d6 for either acc or in the case of spells, damage.
Once everyone participating in the teamwork attack has rolled, you look for paired numbers on the dice.
If any of the numbers match, the attack in considered a success (so long as the rolls being matched are high enough to hit the target in the case of non magical attacks).
Examples:
Jona and Pyre declare their next attack will be a teamwork attack. Their target has 13 AVD and Jona has 6 ACC. Jona rolls 2d6 to hit with his sword and Pyre quickcasts Fira and rolls 2d6 for damage. Jona rolls a 1 and a 3 and Pyre rolls a 1 and a 6. Because Jonas roll was not high enough to hit the monsters AVD the teamwork attack fails even though they have matching #'s. The mana is spent on the spell and the spell misses as well as the attack. I believe this is the only way a spell can straight up miss.
This time Jona rolls a 4 and a 6 and Pyre rolls a 1 and a 4. The attack is a success because spells always hit and Jonas attack beat the targets AVD. Apply the benefits of a sucessfull teamwork attack.
Jona rolls a 5 and a 6 Pyre rolls a 3 and a 4. The attack misses and both attacks fail a no #'s matched. Mana is spent.