If you just want to practice and test synergy you c*ready do that with regular duels. As long as both sides have the indicator that they belong to a team(regardless if it's a 1,2 or 3 man team) the duel happens normally with everyone in it.

Yup, as long as both sides have formed a party if the leader of one challenges the leader of the other one it's a duel with multiple people in it.